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ELEMENTS  OF  PHYSICAL 
GEOGRAPHY 


OF 

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Grand  Canyon  of  the  North  Platte  River,  Central  Wyoming.      (U.  G.  Cornell.) 


Elements  of  Physical 
Geography 


BY 
THOMAS  CRAMER  HOPKINS,  Ph.  D 

Professor  of  Geology  in  Syracuse  University 


ov  troXX'  dWa  iroXv 


BENJ.  H.  SANBORN  &  CO. 

BOSTON  NEW  YORK  CHICAGO 


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GENERAL 

/ 1         Copyright,  1908 
By  raOMAS  CRAMER  HOPKINS 


»HK  MASON-HBNRT  PRK88 
STRACU8E.  NBW  YORK 


PREFACE 

There  are  good  text-books  on  physical  geography,  but 
there  are  many  teachers  and  school  departments  not  satis- 
fied with  any  of  them.  The  author  has  endeavored  to  meet 
the  requirements  of  these  teachers  as  far  as  such  needs  could 
be  ascertained.  With  a  subject  as  broad  as  physical  geog- 
raphy there  will  always  be  lack  of  uniformity  in  the  man- 
ner of  presentation,  as  well  as  in  the  subject  matter.  The 
subject  is  one  which  is  undergoing  many  changes,  and  it  is 
possible  that  both  the  teachers  and  the  subject  may  be 
ahead  of  present  text-books  in  many  particulars. 

This  book  is  not  an  experiment.  To  accommodate  the 
many  students  who  were  going  out  to  teach  in  the  schools 
of  the  State,  the  author,  several  years  ago,  attempted  to 
bring  his  courses  in  physical  geography  in  Syracuse  Uni- 
versity into  harmony  with  the  requirements  of  the  Educa- 
tional Department  of  the  State.  It  was  then  that  he 
realized  the  force  of  the  complaints  of  many  teachers  that 
none  of  the  text-books  met  these  requirements.  After  try- 
ing two  of  the  leading  text-books,  he  abandoned  both  and 
prepared  a  text  which  has  been  used  successfully,  in  manu- 
script form,  for  two  years  in  his  own  classes.  Before 
putting  it  in  book  form  he  studied  the  conditions  in  the 
public  schools  of  New  York  and  in  other  states  and  has 
attempted  to  prepare  a  book  to  meet  the  needs  of  teachers 
and  the  Educational  Departments  in  this  country,  not  with 
the  expectation  of  pleasing  all,  but  with  the  confident  hope 
of  meeting  the  needs  of  many  of  those  who  are  dissatisfied 
with  the  present  books. 

It  is  not  important  that  any  class  should  pursue  the 


188509 


vi  PREFACE 

subject  in  the  order  in  which  it  is  presented  in  this  text. 
The  author's  custom  is  to  begin  with  Chapter  II,  because 
his  classes  commence  in  September.  This  season  is  favora-. 
ble  for  field  work,  which  has  to  do  more  with  the  contents 
of  Chapters  II,  III  and  IV  than  with  Chapter  I.  If  the 
work  should  begin  in  midwinter,  or  there  should  be  no  field 
work,  then  the  order  given  might  well  be  followed.  It  is 
expected,  however,  that  each  teacher  will  follow  his  own 
plan,— the  subject  matter  is  divided  into  chapters  for  that 
purpose.  Each  teacher  will  naturally  expand  that  part  of 
the  subject  best  illustrated  by  the  geographic  conditions  in 
the  proximity  of  the  school.  Those  in  the  glaciated  area, 
by  use  of  the  references  can  devote  more  time  to  the  study 
of  glacial  phenomena.  Those  on  the  shore  of  the  ocean,  or 
a  "large  lake,  can  give  more  time  to  shore  features.  Those 
in  the  interior  can  dwell  more  on  the  work  of  streams  and 
ground  water.  Intensify  the  portion  which  the  pupil  can 
best  study  from  Nature. 

Every  class  in  physical  geography  should  have  more  or 
less  laboratory  and  field  work  associated  with  the  text-book. 
One  of  the  functions  of  the  text-book— not  the  only  one,  by 
any  means— is  to  serve  as  a  handbook,  which  the  pupil 
studies  as  an  aid  in  the  interpretation  of  what  he  sees  in 
the  laboratory. 

To  aid  the  teacher  in  his  work,  the  author  has  prepared 
a  small  laboratory  manual  to  accompany  this  text.  The 
manual  must,  of  necessity,  be  a  book  of  suggestions  rather 
than  directions.  The  work,  to  be  successful,  depends  on 
the  skill  and  tact  of  the  teacher  in  getting  the  pupils  to 
study  and  work  wfth  real  things  rather  than  words  about 
them.  Yet  the  author  believes  that  a  good  book  is  as  much 
needed  and  fully  as  important  in  the  work  of  the  laboratory 
as  in  that  of  the  class-room.    Many  teachers  have  not  had 


PREFACE  vii 

the  opportunity  to  develop  a  systematic  course  of  labora- 
tory work,  consequently  much  time  has  been  wasted  by  the 
pupils  in  routine  work.  The  laboratory  manual  aims  to 
help  the  teacher  and  pupil,  by  suggestions  and  questions, 
to  a  knowledge  of  the  earth  features  and  relations. 

The  text  and  manual  together  aim  to  assist  both  teacher 
and  pupil  into  the  spirit  of  one  of  the  most  inspiring  sub- 
jects in  our  schools;  to  bring  the  pupil  into  contact  with 
Nature  in  such  a  way  that  he  may  see  and  realize  his  own 
position  in  this  w^orld  of  complex  activities,  so  that  by 
observing  more  closely  the  familiar  phenomena  surround- 
ing him  in  his  daily  life  he  may  extend  his  observations 
and  knowledge  through  the  less  known  into  the  unkpown, 
and  thus  be  an  intelligent  part  of  the  great  world  in  which 
he  lives. 

The  author  is  indebted  to  many  teachers  of,  physical 
geography  in  different  states  in  the  preparation  of  this 
text.  After  the  manuscript  was  written  H  was  submitted 
to  a  number  of  prominent  teachers  in  high  schools,  acad- 
emies and  colleges  for  criticism,  and  the  valuable  sug- 
gestions made  by  them  are  incorporated  as  far  as  possible. 
Especially  does  he  desire  to  express  his  indebtedness  to  the 
following  eminent  teachers  for  valuable  aid :  Professor  C. 
E.  Peet,  Lewis  Institute,  Chicago;  Miss  Mary  G.  Sullivan, 
Buffalo  High  School;  Dr.  C.  H.  Richardson,  the  author's 
colleague  in  Syracuse  University;  Miss  Jennie  T.  Martin, 
City  Schools,  Washington,  D.  C. ;  Professor  James  H. 
Smith,  Chicago  High  School;  Dr.  F.  H.  H.  Calhoun, 
Clemson  College,  S.  C. ;  Sarah  Emerson  Green,  formerly 
the  author's  assistant  at  Syracuse  University;  and  P.  F. 
Schneider  of  Syracuse.  The  first  three  above  named  read 
both  the  manuscript  and  the  proof  with  painstaking  care, 
and  the  others   gave  valuable   aid  in  reading  either  the 


viii  PREFACE 

proof  or  the  manuscript.  Dr.  H.  A.  Peck,  Professor  of 
Astronomy,  gave  many  valuable  suggestions  on  Chapter  I, 
and  Morgan  R.  Sanford,  local  forecaster  for  the  U.  S. 
Weather  Bureau,  did  the  same  in  Chapter  X. 

For  the  photographs  illustrating  the  text  the  author  is 
deeply  indebted  to  many  friends  and  colleagues  who  are 
credited  elsewhere.  Special  thanks  are  due  to  the  U.  S.  Geo- 
logical Survey,  the  U.  S.  Fish  Commission,  the  Maryland 
and  Vermont  State  Geological  Surveys,  and  the  American 
Museum  of  Natural  History.  Where  not  otherwise  credited 
the  photographs  are  by  the  author,  except  a  very  few 
where  the  photographer  is  not  known.  The  illustrations 
and  explanations  of  the  same  form  a  very  important  part 
of  the  text  and  should  be  studied  as  carefully  as  the 
words.  In  some  instances  the  picture  illustrates  the  text, 
in  others  the  text  is  an  explanation  of  a  principle  best 
learned  from  the  picture. 

T.  C.  H. 

Syracuse  University, 
May,  1908, 


CONTENTS 


CHAPTER 


PAGE 


I.  The  Earth  as  a  Planet 1 

II.  Groundwater    and    Rivers .........  o  ......  .  40 

III.  Lakes,   Swamps  and  Waterfalls 100 

IV.  Glaciers = 138 

V.  The  Ocean 169 

VI.  Shore   Lines 197 

VII.  The  Land— Minerals,  Rocks  and  Soils.  .....  235 

VIII.  Physiographic   Agencies 274 

IX.  Physiographic   Features    .................<  309 

X.  The    Atmosphere    ...... 348 

XL  Geography  of  Plants,  Animals  and  Man ....  400 

XII.  Physiographic  Regions  of  the  United  States  449 

Appendix   , 473 


CHAPTER  I 

THE  EARTH  AS  A  PLANET 

Introductory.— Physical  geography  literally  means  a 
description  of  the  natural  features  of  the  earth.  The  de- 
velopment of  the  subject  during  recent  years  has  led  to  the 
subdivision  as  follows : 

1.  The  earth  as  a  globe  or  planet,  its  origin  and  rela- 
tion to  the  other  heavenly  bodies. 

2.  The  atmosphere  or  the  surrounding  gaseous  portion. 

3.  The  hydrosphere  or  the  water,  including  the  fresh 
water  and  the  salt  water  of  the  ocean. 

4.  The  lithosphere  or  the  soli.d  land  portions,  the 
causes  producing  the  various  topographic  forms  and  the 
effects  of  these  on  climate  and  life. 

5.  Life  geography  or  the  effect  of  physical  environ- 
ment upon  life  and  its  effect  on  the  earth  features. 

Physical  geography  includes  a  study  of  these  subjects  with 
reference  to  their  influence  upon  man,  his  industries,  civiliza- 
tion and  relation  to  his  surroundings.  With  this  aim  in  view 
it  leads  one  within  the  doorway  of  each  of  the  natural  sciences. 

To  the  ancient  philosopher's  maxim,  "Know  thyself,"  the 
modern  scientist  adds  "in  relation  to  Nature."  This  is  the  foun- 
dation of  modern  physical  geography.  In  gaining  this  knowl- 
edge man  is  better  able  to  adapt  himself  to  his  surroundings,  to 
utilize  the  various  forces  and  products  of  Nature,  to  realize  not 
only  his  dependence  upon  his  fellow  man  and  the  lower  forms  of 
life,  but  his  duty  towards  them  as  well,  and  consciously  or  un- 
consciously, he  must  gain  respect  if  not  love  for  the  Omnipotent 
Power  that  rules  over  all. 

Physical  Geography  is  the  science  which  treats  of  the 

1 


2  PHYSICAL    GEOGRAPHY 

natural  features  of  the  earth  in  their  relation  to  man  and 
the  lower  forms  of  life. 

1.  The  Earth  a  Part  of  the  Solar  System.— The  earth 
is  a  nearly  round  ball  consisting  of  a  large  rock  mass  partly 
covered  with  oceanic  waters,  and  entirely  surrounded  by 
the  gaseous  atmosphere.  The  whole  mass  solid,  liquid  and 
gaseous,  rotates  on  its  axis  as  it  revolves  in  space  around 
the  sun.  It  is  but  one  of  a  number  of  similar  bodies  called 
planets  and  is  in  no  wise  conspicuous  among  them.  It  is 
neither  the  largest  nor  the  smallest;  neither  the  farthest 
from  nor  the  nearest  to  the  sun.  Because  we  live  on  the 
earth,  it  is  most  important  to  us,  but  if  we  could  look  on  it 
from  some  distant  point  in  the  heavens  we  should  not  see 
anything  to  distinguish  it  particularly  from  the  other 
planets. 

2.  What  the  Solar  System  Comprises.— The  earth  is 
an  important  member  of  the  solar  system  which  includes 
the  sun  at  the  center,"  the  planets  and  their  satellites,  the 
planetoids  or  asteroids,  and  some  comets.  Besides  the  earth 
there  are  revolving  around  the  sun  seven  other  planets 
which  are  named  in  order  beginning  with  the  one  nearest 
the  sun, — Mercury,  Venus,  Earth,  Mars,  Jupiter,  Saturn, 
Uranus  and  Neptune.  Four  of  these,  Venus,  Mars,  Jupiter 
and  Saturn,  are  plainly  visible  at  certain  periods.  Two  of 
them,  Venus  and  Jupiter,  are  at  times  the  brightest  bodies 
in  the  heavens  except  the  sun  and  moon.  Part  of  the  time 
they  are  morning  stars ;  at  other  times  evening  stars.  The 
planets  may  be  distinguished  from  the  true  stars  by  their 
steady  light.  The  stars  twinkle.  Each  of  the  planets,  ex- 
cept Mercury  and  Venus,  has  one  or  more  satellites  or 
moons  revolving  around  it.  Saturn  has  besides  the  satel- 
lites several  concentric  bright  rings  surrounding  it.  The 
relative  sizes,  distances  and  other  data  concerning  the 
planets  are  given  in  Appendix  I. 


THE.  EARTH  AS  A  PLANET  3 

The  asteroids  or  planetoids,  about  600  in  number,  are  solid 
bodies  much  smaller  than  the  planets,  and  revolve  in  orbits  be- 
tween Mars  and  Jupiter.  One  of  the  planetoids,  Eros,  about  20 
miles  in  diameter,  discovered  in  1898,  has  a  very  eccentric  orbit 
that  sometimes  brings  it  within  13 1/^  million  miles  of  the  earth. 
The  student  should  learn  to  recognize  the  larger  planets  and  ob- 
serve their  movements  among  the  stars  from  season  to  season. 


^llE  SOLAR  SYSTUA, 


Fia.    1.     The  solar   system,   showing  the   order   of   the   planets,   satellites, 
asteroids,  and  the  orbits  of  a  few  comets. 

3.    Relation  of  the  Solar  System  to  the  Universe.— 

The  Solar  System,  large  and  complex  as  it  appears,  is  but 


4  PHYSICAL    GEOGRAPHY 

one  of  a  number  of  similar  systems  in  the  universe.  Most 
of  the  bright  stars  in  the  heavens  are  suns  similar  to  ours. 
They  appear  to  be  much  smaller  than  our  sun,  but  that  is 
because  they  are  so  much  farther  away.  In  reality  many 
of  them  are  much  larger.  They  probably  have  planets, 
satellites,  comets,  etc.,  like  our  own  system,  but  they  are  so 
far  away  that  these  bodies,  if  they  exist,  are  not  visible 
from  the  earth.  It  is  not  known  how  many  of  these  sys- 
tems there  are,  nor  how  far  out  in  space  they  extend,  but 
certainly  a  great  distance  beyond  our  comprehension.  It 
is  estimated  that  with  a  large  telescope  one  can  see  between 
100  and  200  millions  of  stars,  a  large  per  cent  of  which  lie 
in  the  Milky  Way.  With  few  exceptions  all  of  these  stars 
are  so  far  aw^ay  that  it  takes  the  light  from  them  travelling 
at  the  rate  of  186,000  miles  a  second  many  years  to  reach 
the  earth.  The  moon  is  about  240,000  miles  away  or  about 
ten  times  the  distance  around  the  earth ;  the  sun  is  nearly 
400  times  farther  than  the  moon;  and  the  nearest  fixed 
star  or  neighboring  sun  system  is  several  thousand  times 
farther  than  the  sun.  The  light  of  the  sun  takes  about 
8  minutes  to  reach  the  earth.  The  light  of  the  nearest  star 
takes  31/2  years  to  cross  the  space  separating  it  from  the 
earth.  Truly  the  earth  is  a  very  small  part  of  the  solar 
system  and  an  exceedingly  minute  portion  of  the  universe. 
4.  The  Moon.— The  earth  has  one  satellite,  the  moon, 
which  revolves  around  it  once  a  month  (27.32  days)  and 
accompanies  it  through  space  in  its  journey  around  the  sun. 
The  moon  is  2,163  miles  in  diameter  and  at  an  average 
distance  of  238,840  miles  from  the  earth,  but  it  varies  from 
221,600  to  252,970  miles.  It  is  because  of  its  nearness  to 
the  earth  that  it  is  held  in  its  orbit  around  the  earth  in- 
stead of  pursuing  an  independent  course  around  the  sun. 
(The  synodic  month  or  the  time  from  full  moon  to  full 
moon  is  29.53  days,  but  the  sidereal  month  is  27.32  days.) 


THE  EARTH  AS  A  PLANET  5 

5.  The  Phases  of  the  Moon.— The  moon  emits  no  light  of  its 
own.  All  the  light  that  comes  from  it  to  the  earth  is  reflected 
sunlight.  When  the  moon  is  in  that  part  of  its  orbit  nearest  the 
sun,  it  is  nearly  between  the  earth  and  the  sun,  and  we  see  but 
a  mere  fringe  of  illumination;  it  is  then  the  new  moon.  The 
sunlight  reflected  from  the  earth  faintly  illuminates  its  dark  side 
giving  what  is  called  the  earth  shine.  When  it  has  completed  a 
fourth  of  its  circuit  after  new  moon,  it  is  at  right  angles  to  a 
line  connecting  the  sun  and  the  earth,  and  we  see  one-half  of  the 
illuminated  face,  that  is,  a  fourth  of  the  whole  surface,  and  the 
phase  is  called  the  iirst  quarter.  When  it  has  completed  half  a 
circuit  and  is  on  the  opposite  side  of  the  earth  from  the  sun,  it 


Fig.    2.     The    phases    of    the    moon. 

is  full  moon.  At  the  third  quarter  the  moon  has  completed  three- 
fourths  of  its  circuit  and  one-fourth  of  the  whole  surface  is  again 
reflecting  light  to  the  earth.  The  line  separating  the  illuminated 
portion  from  the  dark  portion  is  called  the  terminator.  Draw 
from  observation  a  figure  of  the  moon  showing  the  light  and  dark 
portion  every  second  night  from  one  new  moon  to  the  next; 
arrange  them  in  order  around  an  ellipse  and  compare  them. 

6,  The  Sun. — The  sun  is  the  center  of  the  solar  sys- 
tem. All  the  planets  of  the  system  revolve  about  it  and 
receive  heat  and  ligrht  from  it.     It  is  much  larger  than  all 


6 


PHYSICAL    GEOGRAPHY 


the  planets  combined,  having  a  diameter  of  866,000  miles, 
which  would  make  it  a  million  times  the  bulk  of  the  earth ; 
but  since  its  density  is  less,  it  has  only  332,000  times  the 
mass  of  the  earth.  Imagine  the  earth  at  the  center  of  the 
sun  and  the  moon  revolving  around  it  in  an  orbit  the  same 
size  as  the  present  one ;  the  moon  would  then  be  about  half 
way  from  the  center  to  the  circumference  of  the  sun. 
7.  The  Sun's  Energy.— Nearly  all  the  heat,  light,  and 
other  forms  of  energy  on  the  surface  of  the  earth  come 
directly  or  indirectly  from  the  sun.     The  radiant  energy 

from  the  sun,  known  as 
insolation,  is  thought  to 
pass  from  the  sun  to  the 
earth  unaffected  by  in- 
tervening space  until  it 
reaches  the  earth  where 
part  of  it,  the  part  that 
we  recognize,  is  percep- 
tible as  heat  and  light. 
The  part  of  the  sun's 
insolation  received  by 
the  earth  is  an  exceed- 
ingly small  part  of  the 
whole,  and  when  one 
realizes  that-  nearly  all 
forms  of  heat  and  light  come  from  the  sun,  the  total 
quantity  radiated  into  space  is  something  beyond  compre- 
hension. All  the  energy  used  by  man  in  heating  and  light- 
ing, all  that  is  used  in  running  machinery  everywhere,  -11 
that  is  used  in  lifting  the  waters  of  the  sea  to  the  clouds  to 
fall  as  rain,  all  that  wonderful  vital  energy  manifested  in 
animals  and  plants,— all  of  these  and  probably  other  forms 
of  energy  as  yet  unrecognized  are  flashed  like  wireless  tele- 
grams across  the  vast  space  that  separates  us  from  the  sun. 


Fig.  3.  Showing  the  relative  size  of  the 
sun  and  the  moon's  orbit.  What  is 
the  scale  of  the  diagram? 


THE  EARTH  AS  A  PLANET 


8.     Eclipses.— Since  the  sun  is  the  source  of  the  light 
received  by  the  earth  and  the  moon,  when  either  of  these 


Annular  EclipM 

Fig.    4.      Solar    and    lunar    eclipses, 

latter  bodies  comes  between  the  other  and  the  sun,   the 
light  of  the  sun  will  be  cut  off.     The  shadow  thrown  by 


8  PHYSICAL    GEOGRAPHY 

the  intervening  body  on  the  other  is  known  as  an  eclipse. 
The  shadow  of  the  moon  on  the  earth  produces  an  eclipse 
of  the  sun,  and  the  shadow  of  the  earth  on  the  moon  causes 
an  eclipse  of  the  moon.  If  the  moon  passes  entirely  into 
the  earth's  shadow,  there  is  a  total  eclipse  of  the  moon,  if 
only  part  of  it  passes  into  the  shadow,  a  partial  eclipse  is 
the  result.  There  may  be  three  kinds  of  solar  eclipses :  ( 1 ) 
a  total  eclipse  when  the  moon  passes  centrally  over  the  disc 
of  the  sun  and  so  near  the  earth  that  the  shadow  reaches 
the  earth;  (2)  an  annular  eclipse,  produced  when  the  moon 
passes  centrally  over  the  disc  of  the  sun  but  is  so  far  from 
the  earth  that  the  end  of  the  shadow  does  not  reach  the 
earth ;  then  the  moon  appears  as  a  black  spot  in  the  center 
of  the  sun  surrounded  by  a  ring  of  light  which  gives  the 
name  annular  or  ring  eclipse ;  ( 3 )  a  partial  eclipse  of  the 
sun  produced  when  the  moon  passes  a  little  to  one  side  of 
the  line  joining  the  earth  and  the  center  of  the  sun. 

If  the  moon  revolved  about  the  earth  in  the  plane  of  the 
earth's  orbit,  there  would  be  a  total  eclipse  of  the  moon  and  sun 
once  each  month,  but  since  the  plane  of  the  moon's  orbit  is  in- 
clined at  an  angle  of  five  degrees  to  that  of  the  earth's 
orbit,  there  is  an  eclipse  only  when  the  moon  passes  one  of  the 
nodes,  that  is,  the  points  of  intersection  of  the  two  orbits,  at  or 
near  new  moon  or  full  moon.  There  may  be  an  eclipse  of  the  sun 
when  there  is  none  of  the  moon,  and  there  must  be  at  least  two 
solar  eclipses  each  year.  Consult  the  almanac  for  several  years 
and  see  how  many  eclipses  of  each  kind  there  have  been. 

In  1907  there  were  four  eclipses,  two  of  the  sun,  one  total 
and  one  annular,  and  two  of  the  moon. 

9.  Comets.— Comets  belong  in  part  to  the  solar  sys- 
tem. Several  hundred  of  these  bodies  have  been  seen  from 
the  earth  at  different  times.  Some  of  them  travel  in  ellip- 
tical orbits  which  extend  millions  of  miles  out  into  space 
beyond  the  outermost  planet  in  our  system,  hence  the  period 
of  revolution  is  one  of  many  years.     Many  comets  travel  in 


THE  EARTH  AS  A  PLANET  9 

a  parabola  or  a  hyperbola  and  become  visible  once  as  they 
pass  around  the  sun  and  away  again,  never  to  return. 
Whence  they  come  and  whither  they  go  is  not  known. 
Prom  fig.  5  it  can  be  seen  that  parabolas  and  hyperbolas 
are  curved  lines,  the  ends  of  which  never  meet. 

The  comets  differ  in  size  and  shape  as  widely  as  they  do 
in  their  orbits.  They  are  characterized  by  a  nucleus  or 
denser  portion  surrounded  by  a  nebulous  mass  called  the 
coma  which  streams  out  from  the  nucleus  and  forms  the 
tail.     The  tail  is  single  or  double  and  of  widely  diverse 


Fig.   5.     Ellipse,   parabola   and  hyperbola.     The  last  two  are  diverging  curves 
which    never    meet. 

shapes  and  differs  in  length  from  that  of  the  diameter  of 
the  nucleus  to  a  length  of  100,000,000  miles  or  more.  As 
a  comet  approaches  the  sun,  the  tail  streams  out  behind  it, 
as  it  passes  perihelion  (the  nearest  point  to  the  sun),  the 
tail  streams  out  ahead  of  it,  that  is,  the  tail  keeps  on  the 
opposite  side  of  the  nucleus  from  the  sun.  Celestial  pho- 
tography has  shown  recently  that  the  tails  of  several  com- 
ets have  been  suddenly  broken  into  two  or  more  parts. 

10.     Historical  Comets.— The  comet  of  1680  is  an  important 
one  because  it  was  the  first  whose  orbit  was  determined  by  the 


10  PHYSICAL    GEOGRAPHY 

principles  of  gravitation.  The  computation  was  made  by  Sir 
Isaac  Newton  who  found  that  it  passed  within  140,000  miles  of 
the  sun  travelling  at  the  rate  of  1,332,000  miles  an  hour.  It  had 
a  tail  100,000,000  miles  long. 

Ealley's  comet  (1682)  is  so  called  because  Halley,  a  friend  of 
Newton,  computed  its  orbit  and  thus  identified  it  with  previous 
comets  that  had  appeared  at  intervals  of  75  years.  He  predicted 
that  it  would  make  its  next  appearance  March  13,  1759.  It  passed 
perihelion  within  a  month  of  that  time.  It  appeared  in  1835  and 
is  due  again  in  1910.    Watch  for  it. 

Biela's  comet  (1826)  was  observed  in  the  latter  part  of  Decem- 
ber, 1846,  to  elongate  and  divide  into  two  parts  which  travelled 
in  parallel  orbits  160,000  miles  apart.  When  they  next  appeared 
in  1852  the  two  portions  were  1,500,000  miles  apart.  They  have 
not  been  seen  since. 

The  Comet  of  1882  was  the  most  conspicuous  one  in  recent 
years.  It  approached  the  sun  in  perihelion  close  enough  to  pass 
through  part  of  its  gaseous  envelope.  Daniel's  comet  attracted 
attention  in  the  summer  of  1907. 

11.  Meteors  and  Shooting  Stars.— Meteors  and  shoot- 
ing stars  are  luminous  bodies  which  are  frequently  observed 
in  our  upper  atmosphere  and  are  sometimes  seen  to  strike 
the  earth.  The  luminosity  of  these  bodies  is  thought  to  be 
due  to  friction  against  the  atmosphere  and  that  before 
entering  the  atmosphere  they  are  cold  and  non-luminous. 
Many  of  them  are  dissipated  in  the  upper  atmosphere,  but 
probably  the  fragments  in  the  form  of  invisible  dust  reach 
the  earth  in  the  course  of  time.  Ten  to  twenty  millions  of 
meteors  strike  the  earth's  atmosphere  every  day.  It  is 
thought  by  some  that  the  earth  has  been  formed  by  the 
aggregation  of  such  particles,  which  would  mean  that  un- 
less the  earth  is  losing  matter  in  some  way  it  is  still  increas- 
ing in  weight. 

12.  Meteorites.— Meteors  which  fall  to  the  earth  are 
called  meteorites.  They  vary  in  size  from  very  minute 
fragments  to  bodies  of  many  tons  in  weight.     The  great 


THE  EARTH  AS  A  PLANET  11 

Tent  meteorite  in  New  York  City  which  Peary  brought 
from  Cape  York,  Greenland,  weighs  36.5  tons.  The 
Bacubirito  meteorite  in  Mexico  weighs  about  27.5  tons. 
The  Willamette  meteorite,  shown  in  fig.  6,  weighs  15.6  tons. 
Some  are  composed  of  stone,  some  of  metals  and  some  of 
both.  About  four  out  of  every  hundred  are  nearly  pure 
iron  with  a  little  nickel.  The  source  of  meteors  and 
meteorites  is  not  definitely  known. 


Fig.  6.  Willamette  meteorite,  the  third  largest  known,  found 
near  Oregon  City,  Oregon.  Length  10  ft.,  height  6  ft.  6  in., 
weight  15.6  tons.      (American  Museum  of  Natural  History.) 

THE  ORIGIN   OF  THE   EARTH 

All  material  things  so  far  as  we  know  have  a  beginning, 
a  period  of  growth,  decline,  and  death.  This  is  not  true 
of  matter  itself  but  of  the  forms  which  it  takes.  The  fact 
is  commonly  recognized  in  regard  to  plants  and  animals 
but  is  probably  no  less  true  of  many  inanimate  objects,  ex- 
cept that  the  changes"  go  on  so  much  more  slowly  that  they 
are  frequently  not  recognized.     It  is  now  known  that  the 


12  PHYSICAL    GEOGRAPHY 

hills  are  not  *' everlasting. "  They  may  be  ''rock-ribbed" 
but  they  are  not  as  ''ancient  as  the  sun."  The  mountains 
have  a  beginning,  and  a  period  of  growth,  after  which  they 
begin  to  dwindle  and  gradually  disappear.  So  it  is  with 
the  earth,  the  sun,  and  the  solar  system ;  they  did  not  al- 
ways exist  as  such.     When  and  how  were  they  formed  ? 

13.  The  Nebular  Hypothesis.— Of  the  many  attempts 
to  explain  the  origin  of  the  solar  system  none  has  met  with 
more  favor  than  that  known  as  the  nebular  hypothesis, 
which  assumes  that  at  one  time  all  the  material  in  the  solar 
system  existed  in  the  form  of  a  rotating  mass  of  nebulous 
gas  that  occupied  all  the  space  from  the  center  of  the 
present  sun  out  to  and  beyond  the  limits  of  the  orbit  of  the 
outermost  planet— Neptune.  Under  the  universal  law  of 
gravitation,  by  which  every  particle  of  matter  in  the  uni- 
verse attracts  every  other  particle,  these  gas  particles  were 
attracted  towards  a  common  center.  In  the  course  of  time 
a  portion  of  the  mass  was  separated  in  the  form  of  a  ring, 
or,  as  some  say,  as  an  irregular  mass,  which  in  time,  by  its 
rotation  on  its  own  axis,  formed  a  spheroidal  body  revolv- 
ing around  the  central  mass.  This  was  the  planet  Neptune, 
which  continued  to  revolve  around  the  central  mass  from 
which  in  turn  the  other  planets  and  their  satellites  were 
separated,  the  earth  being  the  sixth  one  and  Mercury  the 
last  one.  The  residual  central  mass  is  the  sun,  which,  ac- 
cording to  the  hypothesis,  is  still  contracting.  The  plan- 
etary masses  probably  separated  from  the  parent  mass 
while  still  in  the  gaseous  condition,  but  continued  to  con- 
tract until  they  became  liquid  and  on  further  cooling,  solid, 
at  least  on  the  surface. 

This  hypothesis,  with  sundry  modifications,  has  been  widely 
accepted  because  it  seemed  to  account  for  so  many  things  about 
the  system.    Recently  many  objections  to  this  explanation  have 


THE  EARTH  AS  A  PLANET  13 

been  raised,  while  another  explanation  has  been  growing  in  favor 
with  some  people  because  it  appears  to  be  free  from  some  of  the 
difficulties  in  the   nebular  hypothesis. 

14.  The  Planetesimal  Hypothesis.*— The  planetesimal 
hypothesis,  although  it  starts  with  a  nebulous  mass,  differs 
radically  from  the  nebular  hypothesis  in  a  number  of 
particulars.  Unlike  the  first,  however,  the  nebula  is  not 
necessarily  a  gas  nor  is  it  highly  heated  and  hence  it  need 


Fig.  7.     A  spiral  nebula  in  Ursa  Major.      (Ritchey,  Yerkes  Observatory) 


not  pass  through  a  liquid  state.  The  hypothesis  starts  with 
a  spiral  nebula,  which  is  one  of  the  most  common  forms  in 
the  sky  at  present.  The  knots  or  denser  portions  in  the 
nebula  are  the  nuclei  of  the  future  planets  and  satellites, 

*  Formulated    by  Professors    Chamberlin    and    Moulton    of    the    University 
of  Chicago. 


14 


PHYSICAL    GEOGRAPHY 


and  the  nebulous  haze  surrounding  the  nuclei  consists  of 
finely  divided  matter  mostly  solid,  possibly  some  liquid  and 
gaseous,  which  is  later  gathered  in  by  gravitation,  and  added 
to  the  nuclei  to  form  the  planets.  All  of  the  material,  first 
in  the  nebula  and  later  in  the  planets  and  satellites,  moves 
about  the  central  mass  in  elliptical  orbits.  It  all  has  a 
double  motion,  first  around  the  central  axis  of  its  own 
nucleus  or  planet,  and  second  around  the  central  sun. 


•  Fig.  8.     The   great  nebula   in   Andromeda.      (Ritchey,   Yerkes   Observatory.) 


The  hypothesis  supposes  a  relatively  slow  growth  of  the 
earth,  as  of  the  other  planets,  with  increasing  temperature 
in  the  central  portions  due  to  gravity.  That  is,  a  large 
body  will  have  greater  pressure  by  gravity  at  the  center 
than  a  small  one  and,  hence,  will  have  greater  heat  induced 


THE  EARTH  AS  A  PLANET  15 

by  the  pressure.  In  a  body  as  large  as  the  earth,  the  gravi- 
tative  attraction  of  all  the  particles  towards  the  center  pro- 
duces an  enormous  pressure  on  the  central  portions,  a  pres- 
sure sufficient  to  produce  heat  and  raise  the  temperature 
of  the  interior. 

For  a  long  time  after  the  earth  nuclues  began  to  grow 
it  was  too  small  to  have  an  atmosphere  or  even  a  hydro- 
sphere, both  of  which  formed  gradually  as  soon  as  the 
planet  was  large  enough  to  hold  them  by  the  force  of 
gravity.  They  would  be  increased  by  the  parts  expelled 
from  the  interior  by  gravity  pressure,  as  well  as  the  parts 
that  would  be  drawn  to  the  surface  of  the  mass  from  the 
surrounding  nebula.  Hence,  the  accretion  of  the  planet- 
esimal  matter  of  the  outer  half  or  more  of  the  earth  would 
be  through  an  atmosphere  and  subject  to  the  action  of 
moisture.  This  hypothesis  likewise  makes  possible  a  much 
longer  period  of  time  in  which  life  was  possible  on  the 
earth  or  in  which  the  conditions  favored  the  existence  of 
life  in  the  initial  stages,  than  does  the  nebular  hypothesis. 

The  principal  points  of  difference  between  the  two  hypotheses 
are,  that  according  to  the  first,  the  earth  passes  from  a  highly 
heated  gaseous  condition  through  a  hot  molten  state  to  the  pres- 
ent solid  condition,  while  according  to  the  second,  the  earth  was 
never  entirely  gaseous,  never  necessarily  molten  and  possibly 
never  much  hotter  than  at  present.  By  the  first,  the  earth  was 
once  larger  than  at  present  and  included  the  moon  which  was 
later  separated  from  it;  by  the  second,  the  earth  was  never  larger, 
probably  not  so  large  in  the  past  as  at  the  present.  By  the  first, 
the  outer  planet,  Neptune,  is  the  oldest  and  the  inner  one.  Mer- 
cury the  youngest;  by  the  second,  the  planets  and  their  satellites 
are  all  of  about  the  same  age,  that  is,  they  were  all  in  process  of 
formation  at  the  same  time.  According  to  the  planetesimal 
hypothesis  the  moon  is  devoid  of  water  and  an  atmosphere  be- 
cause it  is  too  small  to  hold  them  on  the  surface  by  gravity,  and 
not  because  it  is  so  old  that  it  has  lost  them  as  sometimes 
claimed  in  the  nebular  hypothesis. 


16  PHYSICAL    GEOGRAPHY 

15.  The  Shape  of  the  Earth.— The  earth  is  the  shape 
of  a  ball  that  is  flattened  at  the  poles  and  bulged  at  the 
equator,  so  that  the  equatorial  diameter  is  nearly  27  miles 
longer  than  the  polar  diameter.  It  approaches  an  oblate 
spheroid  more  nearly  than  any  other  mathematical  figure. 

Evidence  that  the  earth  has  a  curved  and  not  a  flat  sur- 
face: (1)  Its  shadow  on  the  moon  is  always  a  curved  one. 
Could  this  be  true  of  a  flat  surface?  (2)  New  stars  ap- 
pear in  front  of  the  observer  and  old  ones  disappear  behind 
him  as  he  travels  toward  the  north  or  south.  How  would 
• 


Fig.  9.     Expansion  of  horizon  with  elevation  indicates  curvature  of  the  earth. 

it  be  on  a  flat  surface?  (3)  The  horizon  expands  rapidly 
as  the  observer  ascends  to  higher  altitudes.  Would  this  be 
true  on  a  flat  surface?  (4)  At  sea  the  slender  toprigging 
of  a  vessel  is  visible  farther  than  the  larger  but  lower  hull. « 
Why?  (5)  There  is  a  marked  difference  in  time  with  a 
change  of  longitude;  thus  the  sun  rises  more  than  three 
hours  later  in  San  Francisco  than  it  does  in  Philadelphia, 
and  nearly  nine  hours  later  than  it  does  at  London.  How 
would  it  be  if  the  earth  were  flat?  (6)  The  earth  has  been 
circumnavigated  many  times.  (7)  The  flattening  at  the 
poles  is  indicated  by  the  increased  weight  of  a  body  in  high 
latitudes  over  that  of  the  same  body  at  the  equator,  and  by 


THE  EARTH  AS  A  PLANET  17 

the  greater  length  of  a  degree  of  latitude  near  the  poles. 
(See  sec.  28  and  fig.  16). 

16.  Cause  of  the  Shape  of  the  Earth.— Nearly  200 
years  ago  it  was  shown  that  an  oblate  spheroid  is  one  of 
the  figures  of  equilibrium  for  a  rotating  body,  and  the  de- 
gree of  oblateness  or  flattening  is  due  to  the  rate  of  rota- 
tion. More  recently  it  has  been  shown  that  the  oblateness 
of  the  earth  corresponds  to  the  requirements  of  a  rotating 
fluid  mass  of  the  size  and  rate  of  rotation  of  the  earth. 
This  was  cited  as  evidence  that  the  earth  was  fluid  at  one 
time  in  its  history  before  reaching  its  present  solid  form. 
But  there  are  good  reasons  for  thinking  that  a  solid  earth 
would  take  the  same  shape  after  a  long  period  of  time,  due 
to  the  shifting  of  materials  on  the  surface,  or  according  to 
the  planetesimal  hypothesis  there  would  be  more  material 
accumulate  at  the  equator  than  at  the  poles. 

Gravitation  shapes  the  material  into  a  sphere.  Rotation 
causes  the  flattening  of  the  sphere  into  the  oblate  spheroid.  It 
is  gravitation  that  holds  bodies  on  the  earth,  and  the  force  in- 
creases with  the  mass  of  the  planet.  If  the  earth  were  the  size 
of  the  moon,  bodies  would  have  much  less  weight  on  its  surface. 
Gases  would  be  so  light  that  they  would  fly  off  into  space  and 
there  would  be  no  atmosphere,  hence  no  water,  and  no  life.  If 
the  earth  were  as  large  as  Jupiter,  bodies  on  the  surface  would 
be  correspondingly  heavier. 

17.  Size  of  the  Earth.— The  diameter  of  the  earth 
through  the  poles  is  7,899.6  miles;  through  the  equator 
7,926.6  miles.  Compute  the  circumference,  the  area,  the 
volume  and  the  weight  of  the  earth  in  tons  from  the  follow- 
ing data: 

1.  The  circumference  equais  the  diameter  multiplied 
by  3.14159. 

2.  The  area  equals  the  product  of  the  diameter  by  the 
circumference  of  a  great  circle. 


18 


PHYSICAL    GEOGRAPHY 


3.  The  volume  equals  the  area  of  the  surface  multi- 
plied by  one-third  of  the  radius. 

4.  The  mass  equals  the  volume  multiplied  by  the  den- 
sity. The  mean  density  of  the  earth  is  5.6.  A  cubic  foot 
of  water  weighs  62.5  lbs. 

18.  Problem  of  Eratosthenes.— The  diameter  of  the  earth 
was  not  the  dimension  first  determined,  as  there  is  no  way  of 
measuring  it  directly.  The  part  that  was  actually  measured  was 
an  arc  of  the  circumference.  This  problem  was  first  solved  by 
Eratosthenes  two  centuries  before  the  Christian  era.    He  deter- 


FiG.  10.  Illustrating  the  problem  of  Eratosthenes.  The  sun's  rays,  vertical 
at  A,  are  inclined  7°  12'  to  the  vertical  sz'  at  s,  which  is  the  angle  at 
the  center  of  the  earth,  C,  measured  by  the  arc  AS.  Fifty  times  this  arc 
equals  the  circumference  of  the   circle  or   the  distance  around  the   earth. 

mined  that  Syene  in  Egypt  was  close  to  the  same  meridian, 
hence  to  the  same  great  circle  as  Alexandria.  He  had  observed 
that  at  noon  on  the  longest  day  in  midsummer  the  sun's  rays 
shone  on  the  bottom  of  a  deep  well  at  Syene  in  Egypt. '  What 
inference  could  he  draw  from  this?  He  measured  the  angular 
distance  of  the  sun  from  the  zenith  at  Alexandria  on  the  same 
day  at  noon  and  found  it  equaled  7  degrees  and  12  minutes,  or 
exactly  one-fiftieth  of  a  circle,  which  is  the  same  as  the  angle  at 


THE  EARTH  AS  A  PLANET  19 

the  center  of  the  earth  formed  by  the  radii  from  these  cities. 
Prove  this.  The  distance  between  the  two  cities  had  been 
measured  and  found  to  be  5,000  stadia,  hence  by  multiplying  this 
distance  by  fifty  he  obtained  the  total  distance  around  the  earth 
as  250,000  stadia.  Unfortunately  we  have  no  means  of  knowing 
at  the  present  time  the  length  of  a  stadium  in  any  of  our  units 
of  measurement,  so  that  we  have  no  certain  means  of  comparing 
the  accuracy  of  the  result  obtained  by  Eratosthenes  with  those 
obtained  by  similar  means  in  more  receot  times. 

19.  Structure  of  the  Earth.— The  earth  is  frequently 
divided  for  convenience  of  study  into  three  parts  or  spheres : 
(1)  The  outer  gaseous  envelope,  the  atmosphere;  (2)  the 
liquid  envelope,  the  water  or  hydrosphere  which  nearly 
surrounds  (3)  the  solid  rocky  part,  the  lithosphere,  the 
inner  portion  of  which  is  sometimes  called  the  centrosphere. 
A  fourth  is  sometimes  added  called  the  biosphere,  or  life 
sphere.  These  are  not  true  mathematical  spheres,  nor  are 
they  very  sharply  separated  at  times.  Both  the  hydro- 
sphere and  the  atmosphere  penetrate  the  lithosphere  and 
large  quantities  of  the  atmosphere  are  dissolved  in  the 
hydrosphere  as  well  as  large  quantities  of  water  as  invis- 
ible vapor  in  the  atmosphere.  The  life  sphere  is  confined 
chiefly  to  the  water  and  the  contact  of  the  atmosphere 
with  the  lithosphere.  It  is  scattered  through  the  water 
sphere  to  a  greater  depth,  probably,  than  in  either  the  gas- 
eous or  the  rock  portions,  yet  the  greater  portion  of  it  lies 
close  to  the  lower  portions  of  the  atmosphere.  The  air, 
water  and  land  portions  of  the  earth,  which  form  the 
greater  part  of  the  subject  of  physical  geography  or 
physiography,  are  discussed  in  the  following  chapters. 

20.  Motions  of  the  Earth.—The  earth  has  (1)  a  daily 
rotation  on  its  axis,  and  (2)  a  yearly  revolution  around 
the  sun.  Besides  these,  it  has  (3)  an  onward  motion 
through  space  in  company  with  the  other  parts  of  the  solar 
system,  but  this  is  not  so  apparent  as  the  other  two  and  is 


^l^ 


20  PHYSICAL    GEOGRAPHY 

not  marked  by  such  pronounced  effects.  There  are  a 
number  of  other  minor  motions  of  interest  to  the  astron- 
omer. 

21.  Rotation.— The  rotation  of  the  earth  on  its  axis 
causes  the  sun  and  the  stars  to  appear  to  revolve  about  the 
earth,  the  sun  appearing  to  rise  in  the  east  and  set  in  the 
west,  producing  the  successive  changes  of  day  and  night 
and  thus  giving  the  measure  of  time,  the  day.  The  rota- 
tion of  the  earth  is  one  of  the  factors  along  with  others  in 
producing  the  tides,  the  belts  of  planetary  winds  and 
calms;  and  it  affects  the  direction  of  the  ocean  currents. 
The  rotation  also  produces  the  bulging  at  the  equator  and 
the  flattening  at  the  poles.  It  causes  a  deflection  of  fall- 
ing bodies.  A  ball  dropped  from  the  top  of  a  tower  would 
be  deflected  to  the  east  of  the  base  of  the  tower,  instead  of 
falling  directly  vertical.  The  deviation  is  greatest  at  the 
equator  and  zero  at  the  poles.  AVhy?  In  the  latitude  of 
New  York  it  is  about  1  inch  for  a  fall  of  500  feet. 

22.  Foucault's  Pendulum.— In  the  middle  of  the  last  century 
Foucault  demonstrated  the  rotation  of  the  earth  by  means  of  a 
pendulum  consisting  of  a  heavy  weight  suspended  on  a  long, 
slender  cord  which  is  started  to  swing  due  north  and  south 
across  a  plane  surface  covered  with  fine  sand.  Attached  to  the 
bottom  of  the  pendulum  is  a  sharp  point  which  traces  a  mark  in 
the  sand  as  it  swings.  If  the  earth  were  still,  the  pendulum 
would  continue  to  swing  on  this  line  but  the  rotation  causes  this 
plane  under  the  pendulum  to  rotate  to  an  extent  varying  with 
the  latitude,  from  zero  at  the  equator  to  a  complete  revolution  at 
the  poles.  This  pendulum  is  still  in  use  at  the  Pantheon  in  Paris 
where  visitors  may  see  the  rotation  taking  place  as  Foucault  did 
in  1851. 

23.  Directions.— The  terms  north,  south,  east  and 
west  are  used  to  signify  directions  on  the  surface  of  the 
earth  and  also  in  space.  North  with  reference  to  the 
earth  really  means  the  direction  of  the  north  pole,  one  end 


THE  EARTH  AS  A  PLANET  21 

of  the  axis  of  the  earth,  and  would  be  a  curved  line  cor- 
responding to  the  meridian  at  the  point  where  the  direc- 
tion is  taken.  What  we  really  think  of,  however,  is  the 
line  on  the  plane  of  the  horizon  which  marks  its  intersec- 
tion with  the  plane  of  the  meridian.  North  in  the  heavens 
refers  to  the  direction  of  the  axis  of  the  earth  prolonged 
to  infinity  passing  nearly  through  the  north  star.  At  a 
point  on  the  equator  this  direction  would  be  identical  with 
north  on  the  earth,  but  as  one  approaches  the  north  pole 
the  two  lines  diverge  until  near  the  pole  they  are  at  nearly 
right  angles  to  each  other.  Represent  this  by  a  diagram 
for  (1)  your  latitude,  (2)  the  equator  and  (3)  the  north 
pole. 

At  the  north  pole  all  directions  on  the  horizon  are  south 
and  the  line  to  the  north  star  is  perpendicular  to  the  hor- 
izon. At  all  other  points  on  the  earth,  south  is  the  oppo- 
site direction  from  north  until  one  arrives  at  the  south 
pole  where  there  is  no  south  but  all  directions  are  north. 

East  refers  to  the  direction  on  the  horizon  at  right 
angles  to  the  north  and  south  line  but  which  if  followed 
proves  to  be  a  curved  line.  West  is  the  opposite  of  east. 
The  equator  and  the  parallels  of  latitude  are  east  and 
west  lines  yet  they  are  circles  on  the  globe.  The  terms 
east  and  west  are  used  for  directions  of  rotation  and  revo- 
lution-, thus  the  earth  rotates  toward  the  east  because  any 
one  point  on  the  earth  at  any  instant  is  moving  east  in  the 
plane  of  the  horizon. 

The  plane  of  the  horizon  is  the  plane  perpendicular  to 
the  plumb  line.  The  point  where  the  extension  of  the 
plumb  line  pierces  the  heavens  is  called  the  zenith  and  the 
direction  is  up.  The  point  opposite  the  zenith  is  the 
nadir  and  the  direction  is  down. 

24.  Revolution.— The  revolution  of  the  earth  around 
the  sun  causes  the  latter  to  appear  to  shift  its  position  in 


22 


PHYSICAL    GEOGRAPHY 


the  heavens  from  day  to  day.  lu  connection  with  the  in- 
clination of  the  earth's  axis  the  revohition  also  causes  the 
change  of  seasons.  The  earth  travels  around  the  sun  in 
an  elliptical  orbit  with  the  sun  at  one  focus  of  the  ellipse. 
At  the  nearest  point  (perihelion)  it  is  about  91,500,000 
miles  distant;  at  the  most  remote  point  (aphelion)  it  is 
about    94,500,000    miles    away.     The    inclination    of    the 


Fia.  11.  The  seasons.  The  sun  is  about  three  million  miles  nearer  the  earth 
in  January  than  it  is  in  July.  Notice  the  variation  in  light  and  darkness 
through  the  different  seasons. 

earth's  axis  to  that  of  the  axis  of  the  ecliptic  or  the  earth's 
path  around  the  sun  is  23  degrees  27  minutes,  which  means 
that  the  plane  of  the  earth 's  equator  is  inclined  at  the  same 
angle  to  that  of  the  plane  of  the  ecliptic.  This  inclination 
remains  fixed  (or  nearly  so)  with  reference  to  space  and 
distant  stars  in  the  heavens,  so  that  the  axis  of  the  earth 


THE  EARTH  AS  A  PLANET  23 

with  slight  variations  always  points  to  the  same  star  in  the 
heavens,  but  it  causes  the  earth  to  assume  quite  different 
positions  with  reference  to  the  sun,  as  shown  in  fig.  11. 

25.  The  Seasons.— On  December  21st  the  northern 
hemisphere  reaches  its  maximum  inclination  from  the  sun, 
the  vertical  rays  of  the  sun  are  on  the  Tropic  of  Capri- 
corn, their  southern  limit,  and  it  is  then  the  winter  solstice 
(Sun  stands).  The  area  around  the  north  pole  is  in  dark- 
ness, and  it  is  winter  in  the  northern  hemisphere,  while  the 
area  around  the  south  pole  is  in  continual  sunshine  and  it 
is  summer  in  the  southern  hemisphere.  The  opposite  con- 
dition prevails  on  June  21st,  the  summer  solstice,  wheni  the 
northern  hemisphere  is  inclined  toward  the  sun  and  the 
rays  are  vertical  on  the  Tropic  of  Cancer.  It  is  then 
summer  in  the  northern  hemisphere  and  winter  in  the 
southern.  On  March  21st  and  September  23rd  the  axis 
of  the  earth  is  perpendicular  to  the  line  joining  the  center 
of  the  earth  and  the  center  of  the  sun  and  the  sunshine 
extends  from  pole  to  pole  when  the  days  and  nights  are 
equal  (the  equinoxes).     Name  the  corresponding  seasons. 

By  consulting  fig.  11  it  may  be  seen  that  the  winter  in 
the  northern  hemisphere  does  not  come  when  the  earth  is 
farthest  from  the  sun,  but  when  it  is  near  perihelion,  and 
the  summer  season  when  it  is  near  aphelion,  showing  that 
the  few  degrees  difference  in  the  angle  at  which  the  sun's 
rays  strike  the  earth  have  a  greater  influence  on  the  tem- 
perature than  the  three  millions  of  miles  difference  in 
distance.  The  heat  of  summer  and  the  cold  of  winter  are 
increased  by  reason  of  long  days  and  short  nights  in  the 
summer  and  long  nights  and  short  days  in  the  winter. 

It  is  thought  that  the  space  between  the  sun  and  the 
earth  is  exceedingly  cold;  and  that  the  sun's  rays  or  inso- 
lation are  changed  to  heat  only  after  entering  the  earth's 


24 


PHYSICAL    GEOGRAPHY 


atmosphere,   and  very   little   there   until  they   strike  the 
solid  earth. 

If  the  axis  of  the  earth  were  perpendicular  to  the  plane  of 
the  ecliptic  what  would  be  the  effect  on  the  seasons?  What 
would  be  the  effect  on  the  winter  and  summer  in  New  York  State 
if  the  axis  were  inclined  twice  as  much  as  at  present?  The  axes 
of  some  of  the  other  planets  are  inclined  much  more  than  that 
of  the  earth.     (See  Appendix  I.) 

26.    Localization  of  Places,— Latitude  and  Longitude. 

— In  Altoona,  Pa.,  all  the  roadways  running  north  and 

south  are  called  avenues 
and  those  running  east  and 
west  are  called  streets,  and 
both  are  numbered  con- 
secutively. Now  if  the 
place  where  the  numbering 
begins  is  known,  one  only 
needs  to  know  the  number 
of  the  street  and  avenue 
to  locate  any  place  in  the 
city  with  reference  to  any 
other   point.      (Study   fig. 

12). 

The  plan  in  the  above 
city  is  a  modification  of 
that  used  in  locating  places 
on  the  globe  or  on  maps 
representing  a  part  of  the  surface  of  the  globe.  Lines  on  the 
globe  represent  imaginary  ones  on  the  earth  and  those  run- 
ning north  and  south  from  pole  to  pole  are  called  meridians 
of  longitude  while  those  running  east  and  west  around  the 
earth  are  called  parallels  of  latitude,  except  the  one  mid- 
way between  the  poles  which  is  called  the  equator.  Since 
each  one  of  these  lines  is  a  circle  around  the  earth  it  con- 


.g 

Fjth 

St 

llh 

St 

■  a 

3rd 

St 

• 

2nd 

St 

Isl 

?it 

b 

Mai 

^  § 

O 

v> 

3 

CO 

3- 

^ 
? 

or 

? 

< 

> 

< 

> 

IV — 

a 

Fig.  12.  City  streets  on  N-S  and 
E-W  lines,  a  is  at  4th  street  and 
2nd  avenue  N.  E.  or,  from  the 
center  of  the  city,  a  is  2  blocks 
east  and  4  north.  Locate  b,  d, 
and  g  in  the  same  way. 


THE  EARTH  AS  A  PLANET 


25 


tains  360  degrees  and  may  conveniently  be  divided  into 
360  parts,  each  representing  one  degree.  Where  more 
lines  are  desired  each  degree  may  be  divided  into  60  parts 
called  minutes  and  each  of  these  again  divided  into  60 
parts  called  seconds  and  each  of  these  into  as  many  frac- 
tions as  desired.  Not  all  of  these  lines  are  drawn  on  the 
globe;  in  fact  only  a  few  of  them  are  represented,  but  it 
is  understood  that  the  space  between  any  two  may  be  sub- 
divided as  indicated.  It  is  only  necessary  to  understand 
the  method  of  numbering  the  lines  and  the  starting  point 
to  know  the  location  of  any  point  on  the  earth,  when  its 

latitude  and  longitude 
are  known.  Latitude  is 
measured  north  and 
south  from  the  equator 
to  the  poles.  Since  the 
poles  are  90  degrees 
from  the  equator  there 

Fig.     13.     Latitude     and     longitude.     Lo-  ^^n    be    Only    90    dcgrCCS 
cate   points   a,   b,   c,   d,   e,    f,    and   g   the  i      i      •        i 

same    as    in    fig.     12    using    degrees    of  north     latitude     and     the 

latitude   in  place  of   streets  and   degrees  game     number     in     SOUth 
of   longitude    in    place    of   avenues.  . 

latitude. 

27.  Determination  of  the  Latitude.— At  sea  the  latitude  is 
generally  determined  by  finding  the  altitude  of  the  sun  by  means 
of  an  instrument  called  the  sextant.  There  are  several  different 
ways  in  which  it  can  be  determined  on  the  land  without  the  use 
of  a  sextant  or  any  other  expensive  instrument.  The  north  star 
is  very  nearly  vertical  over  the  north  pole,  hence  its  altitude 
over  that  point  is  90  degrees.  At  the  equator  the  north  star 
would  appear  on  the  horizon,  that  is,  its  altitude  would  be  zero. 
Hence  the  altitude  of  the  north  star  above  the  horizon  gives  the 
latitude  of  any  place  in  the  northern  hemisphere,  subject  to  a 
slight  correction.  For  methods  of  finding  the  altitude  of  the 
star,  see  laboratory  exercises. 

At  the  times  of  the  equinoxes,  March  21st  and  September 
23rd,  the  sun  is  on  the  equator,  where  a  person  at  noon  on  the 


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m° 

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5. 

Eni 

nfni 

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Q' 

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26 


PHYSICAL    GEOGRAPHY 


dates  mentioned  would  see  the  sun  directly  overhead  or  at  an 
altitude  of  90  degrees,  and  a  person  at  the  north  pole  at  the  same 
time  would  see  the  sun  on  the  horizon.  Hence  on  the  dates 
mentioned  the  altitude  of  the  sun  at  midday,  when  subtracted 
from  90  degrees,  would  give  the  latitude  of  the  place.  On  any 
other  day  in  the  year,  the  same  method  may  be  followed  by  the 
subsequent  addition  or  subtraction  of  the  sun's  angular  distance 


North  fble 


Soaa/bb' 


Somhfbh 


MtrtJuuta  of  L^n^iAuit 

Fig.  14,  Parallels  and  meridians.  In  the  upper  figure  the  lines  do  not 
meet  at  the  center  of  the  earth  because  the  meridians  are  not  circles; 
the  angles  are  measured  by  the   arc  on   the  surface. 

from  the  equator.  This  can  be  obtained  for  any  day  in  the  year 
by  consulting  a  nautical  almanac.  In  this  method  care  must  be 
taken  to  get  the  altitude  of  the  sun  when  it  is  on  the  meridian  or 
the  true  noriln  and  south  line,  which  may  be  determined  by  means 
of  a  magnetic  needle  or  compass  and  the  correction  made  for  the 
local   magnetic   variation;    or  at   night  by   noting  and  marking 


THE  EARTH  AS  A  PLANET 


27 


carefully  the  direction  of  the  north  star;  or  by  noting  the  direc- 
tion of  the  sun's  shortest  shadow  cast  by  any  vertical  post.  (See 
laboratory  exercises.) 


Fig.  15.  Determination  of  latitude  from  the  north  star;  hh,  hh'  etc.  plane 
of  horizon.  At  the  equator  E  the  star,  is  in  the  horizon,  elevation  and 
latitude  zero.  At  40  N.  latitude  elevation  of  N.  star  is  40°.  Elevation 
of  the   star  at  any  point  equals  latitude  of  the  place. 


28.  Length  of  a  Degree 
were  a  perfect  sphere  the 
degrees  of  latitude  would  be 
of  the  same  length  in  all 
places,  but  since  it  is  bulged 
at  the  equator  and  flattened 
at  the  poles,  the  degree  is  a 
little  longer  at  the  poles, 
being  69.407  miles,  while  on 
the  equator  it  is  68.704  mile». 
(Fig.  16). 

29.  Longitude.  —  Since 
longitude  is  measured  east 
and  west  around  the  earth  it 
is  necessary  to  select  a  begin- 
ning point  which  is  called  the 
prime  meridian.    Any  merid- 


of  Latitude.— If  the  earth 

90^  -TOO 


Fig.  16.  Degrees  of  latitude  are 
longer  at  the  poles  than  at  the 
equator  because  they  are  meas- 
ured by  the  arc  of  the  curve  and 
the  flattening  at  the  poles  makes 
the  arc  approach  that  of  a  larger 
circle  than  at  the  equator.  The 
differences  are  exaggerated  for 
clearness. 


28  PHYSICAL   GEOGRAPHY 

ian  might  be  selected,  but  the  one  commonly  used  by  the 
English  speaking  nations  is  the  one  which  passes  through 
the  Royal  Observatory  at  Greenwich,  England,  and  is  called 
the  meridian  of  Greenwich.  Prom  this  point  the  longi- 
tude is  counted  180  degrees  east  and  the  same  number 
west,  the  two  meeting  on  the  180th  meridian.  Why  are 
there  only  90  degrees  of  latitude  and  180  degrees  of  longi- 
tude ?  Why  couldn  't  the  longitude  be  counted  all  the  way 
around  in  one  direction,  360  degrees  either  east  or  west  ? 

Degrees  of  longitude  are  longest  on  the  equator  (69.652 
miles)  and  grow  shorter  both  north  and  south  from  the  equator 
to  zero  at  the  poles.  A  degree  at  40  degrees  latitude  equals 
53.431  miles  and  at  60  degrees  latitude  it  is  only  34.914  miles. 

30.  Determination  of  Longitude.— Longitude  is  determined 
by  finding  the  difference  in  time  between  the  place  in  question 
and  the  meridian  of  Greenwich  or  some  point  whose  longitude  is 
known.  Since  the  earth  rotates  on  its  axis  once  in  24  hours,  in 
one  hour  a  point  on  the  surface  must  go  l-24th  of  360  degrees 
or  15  degrees,  or  one  degree  in  four  minutes.  Hence  the  differ- 
ence in  time  expressed  in  hours  multiplied  by  15  will  give  the 
difference  in  longitude  expressed  in  degrees.  For  example,  a 
place  two  hours  west  of  Greenwich  is  in  2x15  or  30  degrees  west 
longitude.  Longitude  is  commonly  determined  by  a  chronometer 
or  by  telegraph.  Thus  if  one  has  a  chronometer,  which  records 
Greenwich  time,  it  is  only  necessary  to  determine  carefully  the 
time  by  this  chronometer  when  the  sun  crosses  the  meridian  at 
the  point  to  be  determined  and  multiply  the  difference  between 
this  time  and  12  o'clock  by  15  to  have  the  longitude  of  the  place. 
By  the  other  method  if  a  person  in  Buffalo  should  telegraph  to 
St.  Louis  the  exact  time  wh6n  the  sun  is  on  the  meridian  at 
Buffalo  and  the  person  in  St.  Louis  should  substract  this  from 
the  time  when  the  sun  is  on  his  meridian  and  multiply  the  res- 
suit  by  15,  (if  in  hours,  or  divide  by  4  if  in  minutes)  he  would 
have  the  difference  in  longitude  between  the  two  places.  For 
accuracy  an  addition  or  subtraction  must  be  made  for  the  equa- 
tion of  time  (Sec.  33)  obtained  from  the  Nautical  Almanac. 


THE  EARTH  AS  A  PLANET  29 

TIME 

31.  The  Julian  Calendar.— The  chronology  of  ancient 
history  is  very  confusing  and  uncertain  owing  to  the  lack 
of  any  definite  system  for  recording  time.  Julius  Caesar, 
in  the  year  46  B.  C,  reformed  the  Roman  Calendar  into 
the  Julian  Calendar  by  making  every  fourth  year  contain 
366  days  and  the  three  intervening  365  days  each.  He 
also  changed  the  beginning  of  the  year  from  the  first  of 
March  to  the  first  of  January  and  gave  his  own  nam'C  to 
the  month  of  July  while  August  was  later  named  in  honor 
of  his  successor,  Augustus.* 

32.  Gregorian  Calendar.— The  average  length  of  the 
year  in  the  Julian  calendar  was  365.25  days,  which  is 
about  11  minutes  too  long,  a  difiference  which  became 
manifest  after  several  centuries.  It  was  to  correct  this 
that  Pope  Gregory  XIII,  in  1582,  made  another  change  in 
which  10  days  were  dropped  from  the  calendar,  the  day 
after  March  11th  being  called  March  2l3t.  He  modified 
the  part  in  reference  to  the  leap  years  so  that  the  even 
centuries  are  leap  years  only  when  divisible  by  400;  thus 
the  year  1900  according  to  the  Julian  calendar  would  be 
a  leap  year  and  have  366  days,  but  according  to  the  Gre- 
gorian calendar  it  would  have  365  days.  The  Gregorian 
calendar  was  at  once  adopted  in  the  countries  whose 
church  adhered  to  Rome,  but  it  was  not  adopted  in  the 
United  States  and  England  until  1752,  and  it  has  not  yet 
been  adopted  in  Russia  and  Greece.  Hence  in  the  his- 
tories we  frequently  find  the  letters  0.  S.,  old  style,  refer- 
ring to  the  Julian  calendar  and  N.  S.,  new  style,  for  the 
Gregorian  calendar. 

*  Augustus  did  not  want  his  predecessor's  month,  July,  to  be  longer  than 
his  own  month,  so  he  took  a  day  from  February  and  added  it  to  August. 


30  PHYSICAL    GEOGRAPHY 

33.  The  Day.— The  sidereal  day  is  the  length  of  time 
it  takes  the  earth  to  make  a  complete  rotation  with  refer- 
ence to  a  star,  that  is,  until  the  star  is  again  on  the  same 
meridian. 

The  solar  day  is  the  time  of  rotation  with  reference  to 
the  sun.  Suppose  the  sun  and  a  star  on  the  meridian  at 
the  same  time;  during  the  interval  until  the  star  is  again 
on  the  meridian,  the  sun  will  lack  3  min.  56.55  sec.  of  being 
there  owing  to  the  forward  movement  of  the  earth  in  its 
orbit.  Hence  the  solar  day  is  that  much  longer  than  the 
sidereal  day. 

But  solar  days  are  not  all  the  same  length  owing  to  the 
fact  that  the  earth  moves  more  rapidly  in  some  portions  of 
its  orbit  than  in  others.  Since  it  is  not  possible  to  con- 
struct a  clock  that  will  follow  all  the  variations  of  the  sun 
from  day  to  day,  the  length  of  our  day  is  based  not  on  the 
real  sun  but  on  a  mean  sun,  moving  through  the  heavens 
at  the  average  rate  of  the  true  sun.  This  is  called  mean 
solar  time,  which  is  the  time  measured  by  our  clocks.  The 
difference  between  true  solar  time  and  mean  solar  time  is 
known  as  the  ^^ equation  of  time,''  and  may  be  found  in 
the  almanac  frequently  marked  '^sun  fast"  or  ^'sun  slow.'' 
It  should  be  noted  that  even  the  mean  solar  day  is  actually 
determined  by  computation  from  the  sidereal  day. 

The  civil  day  begins  and  ends  at  midnight  rather  than  at  noon 
as  a  matter  of  convenience.  For  the  same  reason  the  astronomical 
day  begins  at  noon.  It  is  also  a  matter  of  convenience  to  have  a 
fixed  place  where  the  day  changes  or  one  day  leaves  off  and  an- 
other begins. 

The  conventional  day.  It  is  always  apparent  noon  on  the 
meridian  under  the  sun  in  its  apparent  passage  around  the  earth. 
In  imagination  if  we  should  follow  the  sun  from  noon  on  Monday 
around  the  earth  until  we  returned  to  the  starting  point  after 
24  hours,  it  would  have  been  noon  all  the  time  and  the  question 
arises,  "Where  did  we  pass  from  Monday  to  Tuesday?"  This 
place  for  the  change  of  date  was  at  one  time  fixed  at  the  180th 


THE  EARTH  AS  A  PLANET  31 

meridian  east  or  west  from  Greenwich,  that  is,  on  the  opposite 
side  of  the  earth  from  the  prime  meridian,  so  that  vessels  crossing 
this  line  would  add  or  subtract  a  day,  depending  upon  which 
way  they  were  going.  It  was  found  that  the  180th  meridian  ex- 
tended through  groups  of  islands  belonging  to  the  same  nation, 
so  that  it  was  found  advisable  to  shift  it  enough  to  have  it  come 
between  nations  and  yet  vary  as  little  as  possible  from  its  first 
position.  It  is  now  called  the  international  or  intercalary  date  line 
and  is  shown  on  fig.  18.  The  day  which  changes  here  is  known 
as  the  conventional  day. 

The  lunar  day  is  the  interval  between  successive  passages  of 
the  moon  across  the  meridian  and  is  nearly  an  hour  longer  than 
the  solar  day. 

In  both  the  sidereal  day  and  the  astronomical  day,  the  hours 
are  numbered  from  1  to  24,  thus  avoiding  the  repetition  of  A.  M. 
and  P.  M.  This  method  of  numbering  the  hours  is  used  on  the 
railways  in  Canada  and  Spain.  Why  is  it  not  used  in  the  United 
States? 

34.  Standard  Time.— If  every  point  on  the  earth 
kept  its  time  by  the  sun  accurately,  it  would  lead  to  a 
constant  change  of  time  as  one  travelled  east  or  west. 
This  was  found  to  be  so  confusing  on  the  railroads  that 
some  years  ago  a  standard  time  was  adopted,  in  which  in- 
stead of  changing  the  time  every  minute  or  second,  or  at 
every  town,  it  is  changed  only  once  an  hour  and  on  the 
even  hours  from  Greenwich.  Thus  in  the  eastern  United 
States  the  time  is  based  on  that  of  the  75th  meridian  or 
the  one  passing  through  Philadelphia.  Further  west  it  is 
based  on  the  90th  or  St.  Louis  meridian,  the  105th  or  Den- 
ver meridian,  and  the  120th  or  the  one  passing  on  the 
boundary  between  California  and  Nevada.  The  time, 
however,  does  not  change  on  these  meridians,  as  by  so 
doing  it  would  give  the  place  immediately  west  of  the  line 
time  nearly  an  hour  different  from  sun  time;  so,  to  make 
the  difference  from  sun  time  as  little  as  possible,  it  is 
aimed  to  make  the  change  of  time  midway  between  these 


32 


PHYSICAL    GEOGRAPHY 


standard  meridians,  but  on  the  railways  it  is  most  con- 
venient to  make  the  change  at  the  end  of  a  division  which 
is  generally  marked  by  some  large  or  important  city.  (See 
fig-  17). 

The  different  names  given  to  these  time  belts  from 
east  to  west  are  Eastern,  Central,  Mountain  and  Pacific 
time  and  they  are  respectively  5,  6,  7,  and  8  hours  slower 
than  Greenwich  time.  The  accurate  standard  time  is  sent 
regularly  at  twelve  o'clock  each  day  from  the  naval  ob- 


FlG.  17.  Standard  time  belts  in  the  United  States.  These 
belts  continue  at  intervals  of  15°  longitude,  making  24  in 
the  circumference  of  the  earth.  Since  they  count  both 
ways  from  Greenwich,  they  meet  on  the  International  Date 
line.      (Fig.  18.) 

servatory  at  Washington  to  all  the  important  telegraph 
offices  in  the  United  States.  Standard  time  is  used  on  the 
railways  in  most  of  the  European  countries. 

35.  Magnetism.— Magnets  are  bodies  which  have  the 
power  of  attracting  iron  and  being  in  turn  attracted  by 
iron  and  in  a  much  less  degree  the  metals  manganese,  co- 
balt and  nickel.  Magnets  are  natural  and  artificial.  The 
natural  magnet  is  the  mineral  lodestone,  or  magnetite.     A 


THE  EARTH  AS  A  PLANET  33 

piece  of  hardened  steel  may  be  made  into  a  permanent  mag- 
net by  rubbing  it  on  a  piece  of  lodestone  or  better,  by  plac- 
ing it  inside  of  a  coil  and  subjecting  it  to  a  strong  electric 
current.  If  the  piece  of  magnetized  steel  or  piece  of  lode- 
stone  be  now  freely  suspended  on  a  pivot,  it  will  form  a 
magnetic  needle,  which,  properly  mounted,  forms  the 
mariners'  compass.  In  the  absence  of  a  pivot  it  may  be 
floated  on  a  cork  in  a  vessel  of  water. 

36.  The  Earth  a  Magnet.— If  two  compass  needles  are 
brought  near  each  other,  it  will  be  seen  that  the  north  end  of  one 
repels  the  north  end  of  the  other,  but  attracts  the  south  end. 
From  this  and  other  observations  it  is  thought  that  the  earth  it- 
self is  a  great  magnet  and  tnat  the  poles  of  the  great  earth  mag- 
net are  near,  but  not  at  the  north  and  south  poles  of  the  earth. 
The  north  magnetic  pole  lies  in  the  region  north  of  Hudson  Bay 
and  west  of  Baffin  Bay,  Recent  studies  seem  to  show  that  it  is 
not  a  fixed  point  but  an  area  of  considerable  size.  The  north  end 
of  the  compass  needle  points  toward  this  magnetic  pol-9,  not  the 
true  north  pole;  hence  in  northern  Greenland,  the  compass 
needle  points  south  of  west  instead  of  north.  In  only  a  few 
places  does  the  magnetic  needle  point  to  the  true  north  and 
these  points  are  connected  by  lines  known  as  agonic  lines.  At 
all  other  points  the  needle  varies  from  a  true  north  direction, 
which  variation  is  known  as  the  magnetic  variation  or  declination. 
Points  having  the  same  declination  are  connected  by  a  line 
called  an  isogonic  line.  Note  the  position  of  the  agonic  and  iso- 
gonic  lines  on  fig.  18. 

Midway  between  the  magnetic  poles  is  the  magnetic  equator, 
where  a  needle  suspended  freely  lies  horizontal.  As  the  needle  is 
taken  north  from  the  magnetic  equator  the  north  end  dips  below 
the  horizontal  until  a  point  directly  over  the  magnetic  pole  Is 
reached,  where  it  stands  vertical.  A  needle  so  suspended  as  to 
swing  freely  in  a  vertical  plane  is  called  a  dipping  or  dip  needle^ 
and  lines  connecting  points  where  the  angle  of  dip  is  the  same 
are  isoclinal  lines  (see  map  on  fig.  18).  The  isoclinals  appear  to 
be  very  nearly  parallel  with  the  isotherms,  which  would  indi- 
cate some  possible  relation  between  the  earth's  magnetism  and 
the  heat  on  the  surface.  It  is  thought  that  in  some  way  the  ro- 
tation of  the  earth  is  a  cause  or  the  cause  of  its  magnetism. 


34 


PHYSICAL    GEOGRAPHY 


THE  EARTH  AS  A  PLANET  35 

MAPS  AND    MAP   PROJECTION 

The  representation  of  the  different  geographic  features 
of  the  earth's  surface  on  paper  has  been  tried  in  a  great 
many  different  ways  in  order  to  gain  accuracy,  combined 
with  ease  and  rapidity  of  construction  and  economy  of 
duplication. 

37.  Globe. — The  ordinary  globe  shows  all  features 
in  their  true  horizontal  relations  better  than  any  other 
method,  but  it  is  too  expensive  and  too  inconvenient  for 
most  purposes.  A  globe  on  which  the  mountains  and 
plateaus  are  shown  in  relief  and  the  ocean  basins  shown 
by  depression  is  more  realistic  and  likewise  more  expen- 
sive. 

38.  Model.— Next  to  the  globe,  a  model  or  relief  map 
constructed  of  plaster,  clay  or  papier-mache  is  one  of  the 
best  ways  of  showing  the  surface  features.  A  model  may 
be  made  of  the  entire  globe  but  more  commonly  it  is  used 
for  small  portions  that  can  thus  be  shown  on  a  larger  scale. 
The  advantage  of  the  model  is  that  it  shows  vertical  as 
well  as  horizontal  relations,  but  the  objections  are  the  ex- 
pense of  construction  and  duplication  and  the  inconvenience 
in  carrying  about  or  storing  for  reference. 

39.  Maps. — Maps  in  which  the  features  are  shown 
on  the  surface  of  thin  paper  are  the  cheapest  to  make, 
much  more  convenient  to  handle  and  store  away,  and  much 
less  expensive  than  either  globe  or  model.  Hence  thero 
are  hundreds  of  times  as  many  maps  in  use  as  models  or 
globes. 

40.  Projections.— Since  the  earth  is  spherical  in  form 
the  attempt  to  represent  its  surface  on  a  flat  paper  is  at- 
tended with  more  or  less  distortion.  A  curved  area  spread 
out  flat  necessitates  crumpling  in  some  places  and  stretch- 
ing in  others.     To  overcome  this  difficulty  various  methods 


36  PHYSICAL    GEOGRAPHY 

have  been  devised  for  projecting  the  lines  of  the  curved 
surface  upon  a  flat  one.  Some  of  the  various  methods  em- 
ployed are  described  in  Appendix  II. 

41.  Scale  of  the  Map.— The  scale  means  the  ratio  of  the 
distances  between  points  on  the  map  and  the  corresponding 
points  on  the  earth.  It  may  be  given  in  units  such  as,  1  inch 
equals  one  mile  or  by  fractions  1-63360  or  1:63360,  which  means 
that  one  inch  on  the  map  corresponds  to  one  mile  on  the  earth. 
The  scale  used  in  the  construction  of  a  map  depends  on  the  size 
of  the  area  to  be  mapped,  the  purpose  for  which  the  map  is 
wanted  and  the  money  available  for  its  construction.  An  in- 
crease in  the  scale  would  mean  an  increase  in  the  cost  of  con- 
struction. Many  of  the  contour  map  sheets  published  by  the 
United  States  Geological  Survey  are  on  a  scale  of  1:62500,  but 
some  have  a  scale  of  1:125000  and  some  1:250000  while  others 
have  a  smaller  scale.  Maps  of  the  whole  United  States  have  a 
much  smaller  scale,  some  being  1:2,500,000,  some  1:7,000,000  and 
still  others  1:14,000,000.  The  scale  should  always  be  marked  on 
a  map  either  by  ratios  or  graduated  lines  or  in  both  ways,  ex- 
cept on  small  scale  maps  of  large  areas  where  the  latitude  and 
longitude  lines  indicate  the  scale. 

42.  Contour  Maps.— Elevations  and  depressions  on 
the  surface  of  the  earth  may  be  represented  on  the  maps 
in  several  different  ways.  (1)  By  relief  maps  or  models. 
On  the  flat  surface  relief  may  be  shown  by  (2)  shading, 
(3)  hachures  or  broken  lines  and  (4)  contour  lines. 

The  model  or  relief  map  shows  to  the  eye  the  features 
of  relief  better  than  any  plan  yet  devised  to  show  the  same 
on  a  map.  Of  the  different  methods  in  use  of  representing 
relief  on  a  flat  surface,  shown  in  fig.  19,  the  contour  map 
is  superior  for  many  purposes.  The  hachures  and  the 
shading  show  hills  and  valleys  but  they  do  not  show  the 
height  of  the  hills  or  the  depth  of  the  valleys.  The  con- 
tour map  shows  not  only  the  relative  but  the  actual  ele- 
vation of  any  and  every  point  on  the  area. 

The  map  is  constructed  by  drawing  lines  connecting 


THE  EARTH  AS  A  PLANET 


37 


Fig.  19.  Representation  of  an  area  (1)  by  shading,  (2)  by 
contour  lines,  and  (3)  by  hachures.  The  method  by  con- 
tours is  the  only  one  that  gives  actual  elevations. 


38  PHYSICAL    GEOGRAPHY 

all  points  that  have  the  same  elevation  above  sea  level. 
(Any  known  point  may  be  taken  as  a  base,  but  sea  level 
is  generally  taken  for  convenience).  The  number  of  con- 
tour lines  drawn  on  the  map  varies  with  the  regularity  of 
the  slopes,  the  scale  of  the  map,  the  heights  of  the  hills, 
and  the  amount  of  detail  desired  in  the  map.  The  vertical 
distance  between  the  lines  known  as  the  contour  interval 
is  sometimes  2,  5,  or  10  feet  on  a  large  scale  map  of  a 
small  area  with  high  hills.  In  a  mountain  district  the  in- 
terval is  50,  100,  500,  or  1000  feet.  (State  the  reasons 
why  an  interval  of  5  feet  is  used  on  the  Donaldsonville, 
La.,  topographic  sheet,  and  100  feet  on  the  Charleston, 
W.  Va.,  sheet). 

In  the  U.  S.  Geological  Survey  maps  the  contour  lines 
are  printed  in  brown  to  avoid  confusion  with  streams, 
roads,  and  other  lines. 

The  first  contour  line  (which  is  generally  not  in  brown) 
on  any  continent  or  island  area  is  the  one  marking  the 
separation  between  the  land  and  the  sea,  that  is,  the  line 
marking  the  contour  of  the  land.  The  second  line  would 
mark  the  contour  of  the  land  if  the  water  should  rise  10 
feet  (or  the  space  of  the  contour  interval)  and  so  on.  If 
one  considers  the  contour  lines  as  marking  the  water  level 
at  successive  stages,  the  significance  of  the  name  becomes 
apparent.  A  contour  line  never  ends  except  at  the  mar- 
gin of  the  map.  On  a  map  of  an  island  or  of  an  entire 
continent  all  contour  lines  are  continuous. 

43.  The  United  States  Topographic  Atlas.— The  United  States 
Geological  Survey  contour  map  sheets  in  the  topographic  atlas  of 
the  United  States  will  in  time  cover  the  entire  area  of  the 
country  on  a  scale  of  approximately  one,  two  or  four  miles  to  the 
inch.  This  is  one  of  the  most  useful  maps  published  in  this 
country  and,  because  of  its  comparative  accuracy,  great  detail, 
and  small  cost,  its  economic  and  scientific  features  should  be 
known  by  all.    It  shows  not  only  such  topographic  features  as 


THE  EARTH  AS  A  PLANET  39 

rivers,  lakes,  roads,  railroads,  villages,  often  separate  houses, 
ferries,  bridges,  mines,  quarries,  etc.,  but  by  contour  lines  the 
absolute  and  relative  elevation  of  any  and  all  points  on  the  area. 

The  atlas  sheets  are  valuable  aids  in  the  study  of  geography 
and  geology.  Some  of  the  points  which  the  student  can  frequent- 
ly interpret  from  the  contour  map  are:  (1)  The  elevation  of  any 
and  all  points  above  sea  level  and  hence  of  any  point  relative  to 
any  other.  From  one  of  these  maps  covering  your  home  dis- 
trict it  should  be  possible  for  you  to  tell  how  many  feet  your 
house  is  above  or  below  the  school  house.  Try  it.  (2)  The 
steepness  of  the  hillsides.  (3)  The  location  of  the  cliffs.  (4) 
The  extent  of  the  drainage  basins.  (5)  The  topographic  age  or 
place  in  the  cycle  of  erosion,  whether  young,  mature  or  old. 
(6)  The  kind  and  structure  of  the  rocks,  whether  igneous  or 
stratified,  whether  folded,  crumpled  or  not.  (On  some  areas  this 
cannot  be  determined  from  the  map.)  (7)  Whether  river  piracy 
has  taken  place  or  is  taking  place.  (8)  Frequently  the  char- 
acter of  the  climate  can  be  inferred  along  with  the  probable  in- 
dustries carried  on  in  the  region,  and  the  density  of  the  popula- 
tion. 

They  are  serviceable  and  interesting  to  travellers.  One 
learns  in  time  to  select  the  best  roads  from  the  study  of  the 
map,  or  in  a  region  where  roads  are  absent,  to  choose  the  best 
route  for  travel  from  place  to  place. 

The  contour  maps  are  of  great  service  in  laying  out  the 
routes  for  roadways,  railways,  electric  lines,  aqueducts,  pipe  lines 
and  irrigating  ditches. 

For  these  and  other  reasons  contour  maps  should  be  thor- 
oughly studied  so  that  the  student  can  properly  interpret  them. 

REFERENCES 

1.  Elements  of  Astronomy,  F.  R.  Moulton,  The  Macmillan  Co. 

2.  Elements  of  Descriptive  Astronomy,  Howe,  published  by 
Silver,  Burdett  &  Co. 

3.  Manual  of  Astronomy,  Young,  Ginn  &  Co. 

4.  New  Astronomy,  Todd,  American  Book  Co.,  New  York. 

5.  Maps  and  Map  Reading,  Ravenstein,  International  Geog- 
raphy, D.  Appleton  &  Co. 

6.  Text  Book  on  Geology,  Chamberlin  and  Salisbury,  Henry 
Holt  &  Co.,  Vol.  2,  Chap.  1. 


CHAPTER  II 
GROUNDWATER  AND  RIVERS 

44.  Rainfall. — Where  do  the  raindrops  come  from 
and  where  do  they  go?  A  vessel  of  water  exposed  to  the 
air  on  a  dry  day  is  soon  emptied.  The  water  evaporates, 
that  is,  it  passes  into  the  air  as  invisible  vapor.  Evapora- 
tion is  taking  place  over  all  the  oceans,  lakes,  rivers,  and 
moist  land  areas,  sometimes  rapidly,  sometimes  slowly. 
This  is  the  source  of  supply  for  the  rain.  Lowering  the 
temperature  and  pressure  of  the  air  causes  the  invisible 
moisture  to  form  clouds  and  thus  become  visible;  on 
further  decrease  of  temperature  and  pressure,  the  water 
condenses  and  falls  to  the  earth  as  rain  or  snow.  Part  of 
the  precipitation  is  again  evaporated  from  the  surface  and 
goes  back  into  the  air  to  be  again  precipitated  elsewhere; 
part  of  it  flows  off  on  the  surface  directly  into  the  streams 
and  thence  back  to  the  ocean;  part  of  it  sinks  into  the 
earth  and  becomes  groundwater.  Some  that  falls  as  snow 
in  cold  climates  forms  streams  of  ice  called  glaciers  that 
move  slowly  downward  towards  the  sea,  in  some  places 
even  flowing  into  the  sea.     (See  Chap.  IV). 

The  proportion  of  the  total  rainfall  that  is  evaporated  direct- 
ly back  into  the  air  is  variable,  depending  upon  such  factors  as 
the  temperature  of  the  atmosphere,  the  rate  of  the  rainfall,  the 
nature  and  condition  of  the  surface,  and  especially  the  humidity 
or  degree  of  saturation  of  the  atmosphere.  In  a  slow,  drizzling 
rain  a  much  larger  portion  will  sink  into  the  earth  than  in  a 
dashing  rain,  which  on  a  steep  slope  runs  directly  into  the 
stream  channels  and  on  a  flat  surface  stands  in  pools  until  it  is 
partly  evaporated  into  the  air. 

40 


GROUNDWATER  AND  RIVERS  41 

Much  of  the  rain  that  falls  on  sand  or  a  broken  rock  surface 
sinks  into  the  earth  through  the  pores,  while  a  large  part  of  that 
which  falls  on  a  hard  rock  or  baked  clay  surface  runs  off  into 
streams  or  stands  in  pools  on  the  surface. 

45.  Groundwater.— The  portion  of  the  rainfall  that 
sinks  into  the  earth  is  called  groundwater  or  underground 
water.  It  penetrates  all  rocks  to  a  great  depth,  passing 
through  the  cracks,  crevices  and  the  pores  of  the  rock.  It 
moves  down  by  force  of  gravity,  capillarity,  and  pressure. 

A  part  of  the  groundwater  finds  its  way  to  the  surface 
again,  some  of  it  quickly,  some  of  it  after  a  long  period  of 
time  and  some  of  it  remains  below  the  surface  indefinitely. 

Depth  of  groundwater.  The  lower  limit  of  the  ground- 
water occurs  at  depths  of  five  or  six  miles  below  the  sur- 
face, where  the  pressure  from  the  overlying  material  is  so 
great  that  even  the  finest  pores  are  closed  and  the  rock 
becomes  too  dense  for  the  water  to  find  its  way  through  it. 

The  water  tahle.  The  successive  rainfalls  through  the 
ages  past  have  filled  the  rocks  with  water  from  the  lowest 
limit  up  to  a  place  where  there  is  a  balance  between  the 
annual  supply  from  the  rains  and  the  loss  through  escape 
to  the  surface.  This  upper  limit  of  the  zone  of  saturated 
rocks  is  known  as  the  water  table,  the  level  of  permanent 
groundwater  qr  the  permanent  water  plane.  It  serves  to 
mark  the  upper  boundary  of  the  water  zone  or  the  zone  of 
permanent  groundwater,  or  the  zone  of  saturation. 

In  some  places  the  water  table  is  at  the  surface,  in  some 
places  a  few  inches  or  a  few  feet  below  the  surface,  while 
in  others  it  is  several  hundred  feet  below  the  surface. 

The  depth  of  the  water  table  depends  upon  the  surface 
features,  the  climate  and  the  rocks.  It  lies  nearer  the 
surface  in  the  valleys  and  plains  than  it  does  in  the 
mountains  and  plateaus.  One  reason  why  the  former  are 
more  productive  than  the  latter  is  that  in  the  one  in  many 


42  PHYSICAL  GEOGRAPHY 

places  the  water  table  is  near  enough  to  the  surface  to  fur- 
nish moisture  to  the  plant  roots  in  dry  seasons,  whereas  in 
the  other  during  a  prolonged  rainless  season  the  water 
near  the  surface  is  evaporated  and  the  water  table  sinking 
too  deep  to  be  touched  by  the  roots,  the  vegetation  withers 
and  dies. 

The  level  below  which  the  moving  water  does  not  sink  in  dry 
weather  is  the  permanent  water  table.  In  wet  seasons  this  level 
rises  towards,  sometimes  to  the  surface,  to  sink  again  during 
the  dry  season.  This  fluctuating  upper  surface  of  the  water 
zone  that  varies  with  the  seasons  is  known  as  the  temporary 
water  table. 

Aquifer.  While  the  rocks  below  the  water  table  are  satur- 
ated, some  of  the  layers  are  so  fine  grained  and  dense  that  the 
water  can  move  through  them  only  with  extreme  slowness,  while 
other  layers  are  coarse  grained  and  permit  free  movement  of 
the  water.  The  latter  are  called  aquifers  (aqua,  water;  fero,  to 
bear)  and  are  very  important  economically  since  they  are  the 
beds  which  supply  the  wells  and  springs.     (See  figs.  32  to  35). 

46.  Destructive  Action  of  Groundwater.— While  per- 
colating through  the  rocks  the  groundwater  has  in  some 
places  a  destructive,  in  some  places  a  constructive  effect  on 
the  rocks  through  which  it  is  passing.  It  comes  in  con- 
tact with  some  minerals  that  are  soluble  and  takes  them  in 
solution,  and  this  solution  acts  as  a  solvent  for  others.  The 
material  dissolved  is  carried  away  by  the  streams  to  the 
ocean,  leaving  cavities  in  the  rock  from  which  the  material 
was  carried. 

47.  Caves. — Carbon  dioxide,  derived  partly  from  the 
air  and  partly  from  the  soil,  dissolves  limestone  when  it 
is  brought  in  contact  with  this  rock  by  the  percolating 
groundwater  which  likewise  acts  as  the  agent  to  carry 
away  the  material  after  it  is  dissolved.  In  this  way  such 
caves  as  Mammoth  Cave  in  Kentucky,  Luray  in  Virginia, 
Wyandotte  and  Marengo  in  Indiana  and  Howe's  Cave  in 


GROUNDWATER  AND  RIVERS  43 

New  York  are  formed.  (See  figs.  20,  21,  27a,  28  and  29.) 
The  above  are  some  of  the  best  known  caves  in  the 
United  States,  but  there  are  hundreds  of  similar  ones  not 
so  widely  known.  Nearly  every  bed  of  limestone  has  num- 
erous caverns  large  or  small.  In  the  upland  areas  of  cen- 
tral Kentucky  it  is  estimated  that  there  are  not  less  than 


I    ,1         l  \    1   .   ^    "  L        f        L      /  ^_ 


TlG.   20.     Vertical   section    of  limestone    showing   caves    and    sink 
holes.     Notice  the  vertical  cliff  and  talus  slope  at  the  left. 

10,000  miles  of  limestone  caverns.  Limestone  caves  vary 
in  length  from  a  few  feet  to  many  miles,  in  depth  below 
the  surface  from  a  few  feet  to  several  hundred  feet. 

48.  Life  in  Caves. — As  might  well  be  imagined  caves 
frequently  form  retreats  or  hiding  places  for  different 
animals,  the  most  common  being  the  bats,  which  fly  abroad 
in  summer  nights  but  spend  the  days  and  the  winter  season 
in  the  caves  where  they  sometimes  cluster  in  great  masses 
hanging  frofti  the  roof  of  the  cavern.  Beetles,  lizards, 
mice,  wolves,  bears,  and  foxes  are  some  of  the  other  animals 
that  find  a  home  in  the  caves.  Blind  fish  are  sometimes 
found  in  them. 

Savage  man  in  ages  past  found  shelter  there.  Relics  of  his 
handiwork,  his  implements  and  his  carving  on  the  walls,  have 
been  found  in  different  caves  often  associated  with  bones,  and 
pictures  made  by  him  of  now  extinct  animals  and  even  the  bones 
of  primeval  man  himself  have  been  found. 

Corradingf  Action  of  Cave  Streams.— Besides  the  dis- 
solving action  of  the  groundwaters  on  the  rocks  there  is  in 


44 


PHYSICAL  GEOGRAPHY 


Fia.  21  Map  of  portion  of  the  explored  galleries  in  Mammoth  Cave,  Ken- 
tucky. Some  of  the  galkries  are  much  higher  than  others.  There 
is  a  descent  of  many  feet  at  the  entrance,  and  one  crosses  several 
lofty  hills  in  traversing  the  different  chambers.  The  shaded  portions 
contain  water.  This  cave  has  been  explored  through  nearly  200  miles 
of  galleries.     Scale  about  1  V4   miles  to  one  inch. 


GROUNDWATER  AND  RIVERS 


45 


places  a  corrading  or  wearing  action  on  the  bottoms  and 
sides  of  caves  similar  to  the  work  of  surface  streams.  In 
some  places  several  caves  occur  one  above  the  other.  The 
upper  ones  are  dry  while  the  lowest  one  frequently  has 
a  stream  in  part  of  the  cave.  Most  of  the  corrading 
work  done  in  the  cave  is  not  done  by  the  permanent  stream 
but  by  the  temporary  streams  that  pour  in  through  open- 
ings in  the  roof  during  the  rainy  seasons. 


Fig.  22.  "The  Bottomless  Pit."  A  limestone  sink  near  Flagstaff, 
Arizona.  The  sink  is  100  feet  in  diameter.  The  stream  that 
disappears  here  is  not  known  to  reappear  at  the  surface.  In  a 
wet  season  the  water  fills  the  pit,  overflows  and  forms  a  lake. 
The  flat  in  the  picture  is  the  silt-covered  lake  bed  as  it  appears 
during  low  water.      (Hackett.) 

Lost  River  in  Indiana  flows  through  limestone  caverns  for 
about  10  miles  of  its  course,  but  in  flood  season  when  there  is 
more  water  than  can  find  escape  through  the  underground  chan- 
nel the  surplus  flows  in  the  surface  channel  until  the  flood  sub- 
sides when  it  disappears  into  the  cave.  Presumably  there  is 
considerable  corrasion  in  such  a  cave. 

49.  Sink  Holes. — In  nearly  every  limestone  region 
where  there  are  caves  there  are  numerous   basin-like   or 


46  PHYSICAL  GEOGRAPHY 

funnel-shaped  depressions,  called  sink  holes  or  swallow- 
holes.  These  are  often  shaped  like  a  funnel,  the  large 
opening  serving  to  catch  the  rainfall  and  to  lead  it  into  the 
narrow  opening  at  the  bottom,  corresponding  to  the  stem 
of  the  funnel.  Through  the  sink  hole  the  surface  water 
drains  into  larger  caverns.     (See  figs.  20  and  22.) 

A  limestone  surface  much  diversified  by  the  action  of 
the  groundwater  dissolving  the  rock  along  cracks  and  joint- 
planes,  thus  leaving  many  deep  irregular  fissures,  is  called 


Fig.  23.  Surface  of  limestone  outcrop  at  Syracuse  Caves.  The 
openings  were  formed  by  the  ground- water  dissolving  the 
rock  along  the  cracks  and  joint  places.  Some  of  the 
cavities  extend  to  a  depth  of  more  than  100  feet. 

by  the  Germans  the  Karsten.  There  is  no  English  word 
for  this  phenorTDcnon  although  it  occurs  in  many  places  in 
New  York  and  elsewhere  in  the  United  States.  (See  fig.  23.) 
50.  Natural  Bridges.— Natural  bridges  are  formed 
sometimes  by  the  breaking  down  of  part  of  the  roof  over 
one  of  these  subterranean  streams.  The  portion  of  the 
roof  that  remains  spanning  the  now  open  chasm  is  called 


GROUNDWATER  AND  RIVERS  47 

a  natural  bridge.     Natural  bridges  are  sometimes  formed 
in  other  ways.     (See  figs.  24,  25,  146  and  148.) 

51.     Constructive  Action  of  Groundwater.— Some  of 
the  mineral  material  taken  in  solution  by  the  groundwater 


Fia.   24.     Natural   Bridge,   Va.      The  remnant  of  the  roof  of  a 
cave  formed  under  a  waterfall.      (U.   S.   Geol.   Survey.) 

is  carried  in  solution  to  the  ocean  while  part  of  it  is  de- 
posited again,  sometimes  on  the  surface  where  the  ground- 
water emerges,  and  sometimes  underneath  the  surface.     In 


48 


PHYSICAL  GEOGRAPHY 


Fig.  25.  The  Caroline  Bridge,  Utah.  The  longest  natural  bridge  in  the 
world.  Length  350  feet,  width  60  feet,  thickness  60  feet.  Copy  of 
painting  made  from  photographs,  sketches,  and  the  measurements. 
The  rock  is  light  colored  sandstone.       (Courtesy  of  E.   F.  Holmes.) 


Fia.  26.  Small  calcite  veins  in  limestone,  formed  by  ground- 
water carrying  carbonate  of  lime  in  solution  and  depositing 
it  in  cracks  in  the  limestone.      (U.  S.  Geol.  Survey.) 


V^  OP  THE 

UNIVERSITY 


OF 


CALIFOBJi^ 


ROUND  WATER  AND  RIVERS 


49 


percolating  through  the  rocks,  the  water  and  the  carbonic 
acid  gas  in  the  water  are  under  pressure  when  they  take 
up  more  carbonate  of  lime  than  they  can  hold  in  solution 
under  less  pressure ;  hence,  when  the  water  reaches  a  large 
cavity  or  the  surface,  where  the  pressure  is  lowered,  some 
of  the  acid  gas  escapes  into  the  air,  some  of  the  water  is 
evaporated  and  part  of  the  mineral  matter  is  deposited. 
52.  Veins.— When  the  mineral  matter  carried  in  solution  is 
deposited  in  cracks  or  fissures  in  tlie   rocks,  it  forms  veins,  in 


Fig.  27.  Gold-silver  vein  near  Ouray,  Colorado.  The  vein 
is  composed  of  quartz  containing  gold  and  silver.  It 
is  about  20  feet  wide  and  thousands  of  feet  in  depth. 
It  fills  a  deep  fissure  in  the  dark  colored  volcanic 
rock  which  shows  on  each  side  of  the  white  quartz. 
Such  veins  are  called  fissure  veins. 


which  are  formed  compounds  or  ores  of  different  metals  such  as 
gold,  silver,  lead,  zinc,  copper,  etc.  Mingled  with  the  ores  are 
variable  quantities  of  other  minerals  known  as  gangue  or  vein- 
stuf  consisting  of  calcite,  fluorite,  barite,  quartz,  and  other  min- 
erals, all  of  which  are  carried  by  the  groundwaters  into  fissures 
and  there  deposited  to  form  the  vein.  Man  is  largely  dependent 
4 


50 


PHYSICAL  GEOGRAPHY 


upon  these  veins  for  the  supply  of  metals  needed  in  the  different 
industries,  because  in  the  original  condition  of  the  rocks  the 
metals  are  so  scattered  and  diffused  that  they  cannot  be  profit- 
ably extracted  until  they  are  segregated  as  ores  in  the  veins  by 
the  action  of  the  groundwater. 


Fig    27a.      Stalactites,    stalagmites    and    columns    in    Marengo 
Cave,   Ind.      (Hessler  and  Smith.) 


53.  Cave  Deposits.— The  water  that  very  slowly  drips 
from  the  roof  of  a  limestone  cave  is  partly  evaporated  and 
at  the  same  time  permits  the  escape  of  part  of  the  carbon 
dioxide,  which  causes  part  of  the  lime  carbonate  to  be  pre- 
cipitated in  the  form  of  an  icicle-like  deposit  called  a 
stalactite.  A  corresponding  projection  built  up  on  the 
floor  of  the  cave  is  called  a  stalagmite.  How  can  you 
prove   that   these   are   carbonate   of  lime?     Many   of   the 


GROUNDWATER  AND  RIVERS 


51 


stalactites  have  a  small  hole  running  lengthwise  through 
the  middle.     How  do  you  account  for  it? 

If  the  stalactite  and  the  stalagmite  grow  together 
forming  a  continous  deposit  from  the  roof  to  the  floor  it 
is  called  a  column  or 
pillar.  A  growth 
along  the  wall  of  the 
cave  extending  from 
the  floor  to  the  roof 
is  called  a  pilaster. 
In  some  places  this 
deposition  goes  on 
until  the  cave  that 
was  originally  formed 
by  the  groundwater 
is  completely  filled  by 
it.  The  more  mas- 
sive and  compact  de- 
posits formed  in  the 
cave  are  quarried  and 
used  as  onyx  marble 
or  Mexican  onyx.  (See 
figs.  27a,  28  and  29.) 

54.  Spring  Deposits.— The  calcite  or  carbonate  of 
lime  is  frequently  deposited  around  springs,  which  are 
streams  of  groundwater  appearing  at  the  surface.  The 
deposit  of  the  spring  is  formed  similarly  to  that  in  the 
cave,  namely,  by  the  escape  of  the  carbonic  acid  gas  and 
evaporation  of  some  of  the  water,  causing  part  of  the 
dissolved  lime  to  be  deposited.  It  is  frequently  deposited 
on  the  surface  of  moss,  leaves,  or  twigs  because  a  large 
area  is  there  exposed  to  evaporation.  Such  porous  de- 
posits are  called  calcareous  tufa.  The  more  massive  de- 
posits formed  by  springs  and  streams  are  called  traver- 


Fig.  28.  "Tower  of  Babei/  Marengo  Cave, 
Indiana.  Small  stalactites  on  the  roof. 
Stalagmites  on  the  floor.  A  column  ex- 
tending from  roof  to  floor.  (W.  S. 
Blatchley.) 


52 


PHYSICAL  GEOGRAPHY 


tine,  a  name  which  is  sometimes  used  for  all  the  deposits 
of  carbonate  of  lime  from  solution.  The  Coliseum  and 
St.  Peters  and  many  other  large  buildings  at  Rome  are 
constructed  of  travertine,  quarried  from  an  extensive  de- 
posit formed  hy  the  springs  at  Bagni  near  Rome.  ( See  figs. 
30  and  177). 

Other  materials  than  lime  may  be  brought  to  the  surface  by 
the  springs  and  deposited,  such  as  iron  oxide,  sulphur,  and  silica. 
The   silica-depositing  springs  are   generally  hot  springs. 

55.   Induration.— 

Mineral  matter,  such 
as  silica,  and  the  car- 
bonates of  lime  and 
iron  carried  in  solu- 
tion by  the  ground- 
waters, is  sometimes 
deposited  in  the  open 
spaces  between  the 
grains  in  a  bed  of 
sand  or  gravel,  ce- 
menting the  particles 
together  and  thus 
changing  it  into  a 
bed  of  sandstone  or 
conglomerate.  This  is 
one  of  the  principal 
ways  in  which  beds 
of  sediment  are  in- 
durated or  changed  to  solid  rock.  Frequently  the  water 
is  brought  to  the  surface  by  capillarity,  where  it  evaporates, 
precipitating  the  mineral  matter  in  the  pores.  The  fact 
that  many  sandstones  are  harder  on  the  surface  of  the  out- 
crop than  in  the  interior  of  the  bed  is  accounted  for  in 
this  way. 


Pig.  29.  "Pillar  of  the  Constitution"  in 
Wyandotte  Cave,  Indiana.  A  huge 
column  of  calcite,  surrounded  by  small 
stalactites  on  the  roof.  (W.  S. 
Blatchley.) 


GROUNDWATER  AND  RIVERS 


53 


In  many  places  in  the  northern  United  States,  portions  of  the 
glacial  sand  and  gravel  deposits  are  cemented  by  calcite  de- 
posited from  solution  in  the  groundwater.  The  student  may 
readily  test  this  by  placing  a  piece  of  the  material  in  some  dilute 
acid  and  noting  the  rapid  effervescence,  followed  by  the  crumbling 
of  the  piece  into  separate  grains  or  pebbles.  It  is  this  small  per 
cent  of  lime  that  makes  the  glacial  gravels  better  road-making 
material  than  the  gravels  from  the  creek  beds. 


'Fig.  30.     Travertine,  carbonate  of  lime  deposited  by  hot  springs, 
Yellowstone  National  Park.      (Detroit   Pub.   Co.) 


56.    Reappearance  of  Groundwater  at  the  Surface.-— 

What  becomes  of  all  the  groundwater  that  sinks  into  the 
earth?  Part  of  it  is  brought  up  by  capillary  attraction 
and  evaporated  from  the  surface  in  dry  weather.  Part 
of  it  is  brought  up  through  the  roots  and  stems  of  plants 
and  evaporated  from  the  leaves.  Part  of  it  reaches  the 
surface  through  artificial  openings  such  as  wells,  artesian 
wells,  mines,  tunnels,  and  borings.  Part  of  it  combines 
chemically  with  minerals  beneath  the  surface  and  is,  tem- 
porarily, at  least,  locked  up  as  water  of  crystallization. 
The  water  of  crystallization  is  abundant  in  much  of  the 
loose  surface  rock,  in  clay  and  brown  iron  ore.  It 
may  be  detected  by  taking  a  handful  of  clay  soil,  drying 


54 


PHYSICAL  GEOGRAPHY 


it  thoroughly  and  putting  part  of  it  in  a  test  tube.  Heat 
it  over  a  gas  lamp,  when  the  water  of  crystallization  will 
be  separated  and  condensed  as  drops  on  the  side  of  the 
tube.  The  amount  of  water  may  be  determined  by  weigh- 
ing the  sample  before  heating  and  after  heating.  The 
experiment  is  better  performed  with  gypsum  or  limonite. 
Part  of  the  groundwater  penetrates  the  rocks  to  great  depths 
and  may  not  get  back  to  the  surface  for  many  centuries;  pos- 
sibly a  small  part  of  it  may  never  return.  A  considerable  and  very 
important  part  of  the  groundwater  is  returned  to  the  surface 
through  springs  and  seepage,  including  hot  springs  and  geysers. 

57.  Wells  are  openings  dug  or  bored  from  the  sur- 
face down  to  a  short  distance  below  the  groundwater  table 
for  the  purpose  of  obtaining  water;  in  the  ordinary  well 
the  water  stands  as  high  and  no  higher  than  the  water 
table.    In  fact,  the  best  way  to  locate  the  water  table  in  any 


Fig.     81.      Variation     of     the     water     table     with     the     seasons. 

a,  temporary    water   table,    April.      Water   in    all    the    wells. 

b,  temporary  water  table,  June.  W'  is  dry.  c,  permanent 
water  table,  September.  W'  and  W  are  dry.  v',  Young 
valley  with  temporary  stream  in  wet  season  only.  Vo  Larger 
valley  contains  stream  until  the  temporary  water  table 
sinks  below  b.  Va  Mature  valley,  contains  a  permanent 
stream. 

region  is  by  the  level  of  the  water  in  the  wells  when  they 
are  first  opened.  Sometimes  excessive  use  of  the  water 
from  a  few  large  wells  or  many  small  ones  may  cause  a 
lowering  of  the  water  table  that  is  often  of  serious  im- 
portance. 

Every  well  opening  sunk  below  the  water  table  will 
not  prove   productive   because   in  some   places  the  rocks 


GROUNDWATER  AND  RIVERS 


55 


are  so  dense  that  very  little  water  can  find  its  way  through 
into  the  well.  In  porous  rocks  the  wells  are  fed  by  water 
which  seeps  through  the  pores  into  the  well  opening  and 
any  well  sunk  into  an  aquifer  or  porous  layer  below  the 
level  of  the  water  table  will  be  productive.  In  the  denser 
rocks  the  wells  are  fed  by  tiny  underground  streams.  The 
well  that  strikes  one  or  more  of  the  little  streamlets  may 
have  a  bountiful  supply  of  water  while  another  close  by 
that  has  missed  the  streamlets  (or  so-called  veins  of  water) 
may  be  barren. 

The  reason  that  some  wells  go  dry  in  times  of  drought 
is  that  the  water  table  sinks  below  the  bottom  of  the  well. 
Sometimes  the  reverse  is  true  when  the  water  table  rises 
near  or  even  to  the  surface  and  the  well  is  filled  to  over- 
flowing.    (See  fig.  31.) 

58.  The  artesian  well  differs  from  the  common  well 
in  that  it  occurs  only  in  inclined  strata  down  the  slope  of 


Fig.  32.     Artesian  wells  W,  W,  W,  in  which  the  water  supply 
comes  from  the  different  aquifers  A,  A',  A". 

which  the  groundwater  moves  and  enters  the  well  under 
pressure  which  causes   it  to   rise   in   the  well  above   the 

water    plane.      The    water 
may  even  flow  out  of  the 
mouth  of  the  well  which  it 
does  frequently  with  con- 
siderable  force. 
The    name    arte- 
sian    is    derived 

Fig.   33.     A    more    favorable     condition    for     artesian    fpQjjj      Artois       3 
wells  than  that  shown   in   B,   because  the  strata  .  '  . 

are    not    so    highly    inclined.  prOVlUCC        in 


56  PHYSICAL  GEOGRAPHY 

France  where  the  first  well  of  this  kind  was  bored.  It  was 
a  very  strong  flowing  well  and  for  a  long  time  only  flowing 
ones  were  called  artesian,  but  now  the  name  is  used  for  all 
deep  wells  where  the  water  enters  under  pressure  and  rises 
considerably  above  the  point  of  entrance  and  above  the 
water  plane.     (See  figs.  32  and  33.) 

The  necessary  conditions  for  an  artesian  well  are  (1) 
a  layer  of  porous  rock,  the  aquifer,  through  which  the 
water  can  percolate  freely;  (2)  the  strata  inclined  to  the 
horizontal;  (3)  the  porous  layer  outcropping  in  a  region 
of  considerable  rainfall,  and  (4)  the  aquifer  overlain  by 
a  layer  of  rock  less  pervious;  (5)  all  the  strata  dipping 
into  the  groundwater  zone.  It  is  immaterial  what  kind 
of  rock  is  below  the  aquifer;  if  impervious  it  will  hold 
the  water  above  it ;  if  porous  it  will  fill  with  water  and  in 
that  way  become  impervious.  (6)  There  should  be  no 
natural  escape  for  the  water  between  the  outcrop  and 
the  well;  (7)  nor  any  obstruction  to  prevent  the  water 
reaching  the  well.  (Study  the  diagrams,  figs.  32,  34  and 
35.) 

The  most  favorable  condition  for  an  Artesian  well  is  a  gentle 
inclination  of  the  strata  as  in  fig.  34  and  not  highly  inclined,  as 


Fig.  34.  Section  from  the  Black  Hills  across  portion  of  the 
Great  Plains.  The  Dakota  sandstone  is  the  aquifer  which 
receives  the  rainfall  on  its  upturned  edges  in  the  Black 
Hills  and  carries  it  as  groundwater  hundreds  of  miles  out 
under  the  dry  plains  where  it  is  obtained  by  artesian 
borings.      (After  Darton).      See  Fig.    33. 

in  Fig.  33  B  because  in  the  latter  case  the  well  must  be  near  the 
outcrop  in  order  to   reach   the   aquifer  or   water-bearing   layer, 


GROUNDWATER  AND  RIVERS 


57 


while  in  the  first  case  the  well  may  be  many  miles  distant,  even 
in  a  semi-arid  or  desert  region,  and  yet  get  the  water  from  the 
rain  belt  far  away.  Thus  "there  are  artesian  wells  on  the  desert 
Of  Sahara  fed  by  the  rainfall  on  the  bordering  mountains.    The 


Fig.  35.  Flowing  artesian  well  at  Woonsocket,  South  Dakota.  It 
throws  a  3-iiich  stream  to  the  height  of  97  feet.  This  well  is 
about  200  miles  from  the  Black  Hills.  (N.  H.  Darton,  Nat.  Geog. 
Mag.   Aug.    1905.) 


58  PHYSICAL  GEOGRAPHY 

rain  that  falls  on  the  sharp  crested  foothills  of  the  Rocky  Moun- 
tains and  the  Black  Hills  is  carried  in  a  bed  of  sandstone  out 
under  the  great  Western  Plains  for  many  miles  where  it  is  ob- 
tained from  artesian  wells,  in  some  places  even  in  sufficient 
quantities  for  irrigation  purposes.  The  gently  inclined  beds  of 
clay  and  sand  on  the  coastal  plain  of  Long  Island  and  New  Jersey 
favor  productive  artesian  wells,  which  might  even  be  sunk  out 
in  the  ocean  and  furnish  a  bountiful  supply  of  fresh  water.  Draw 
a  diagram  to  illustrate  this.     (See  figs.  35  and  33.) 

59.  Springs. — Much  of  the  groundwater  returns  to 
the  surface  in  the  form  of  springs,  which  are  streams  of 
groundwater  emerging  at  the  surface,  and  varying  in  size 
from  tiny  trickles  to  great  rivers.  Silver  Spring  in 
Florida  and  Mammoth  Spring  in  Arkansas  are  each  large 
enough  to  float  a  small  steamboat. 

Sometimes  the  groundwater  descends  only  a  few  feet 
below  the  surface  until  it  finds  its  way  back  to  the  sur- 
face through  a  spring.  Sometimes  it  descends  thousands 
of  feet  before  it  is  returned. 

60.  Temperature  of  Springs.— The  temporary  springs  feA 
from  water  near  the  surface  vary  in  temperature  during  the  year, 
becoming  warmer  in  summer  and  cooler  in  winter.  The  perma- 
nent springs,  however,  are  fed  by  water  from  below  the  per- 
manent water  table  which  is  generally  below  the  zone  of  variable 
temperature,  and  the  water  has  a  uniform  temperature  through- 
out the  year,  generally  about  55  degrees  F.  (Compare  the  tem- 
perature of  the  water  in  some  of  the  springs  in  your  neighbor- 
hood in  the  summer  and  in  the  winter  months.  Why  does  the 
water  seem  colder  in  the  summer  and  warmer  in  the  winter?) 
Some  springs  in  a  rocky  region  have  a  temperature  considerably 
below  55  degrees  because  part  of  the  water  comes  from  melting 
ice  whicn  accumulates  in  the  talus  slopes  during  the  winter  and 
melts  slowly  during  the  summer.  Sometimes  the  ice  forms  in 
limestone  caves.  There  are  several  of  these  caves  in  the  lime- 
stone near  Syracuse,  N.  Y. 

61.  Hillside  Springs.— The  very  large  springs  gen- 
erally occur  in  the  bottom  of  the  valley.     Why?     But  a 


GROUNDWATER  AND  RIVERS 


59 


great  many  small  and  medium  sized  springs  emerge  at 
different  elevations  on  the  hillsides,  frequently  a  number 
of  them  at  the  same  level.     These  are  known  as  hillside 


Fig.  36.  Hillside  Springs  on  an  area  of  five  square  miles  at 
Eureka  Springs,  Arkansas.  The  shaded  portion  repre- 
sents a  porous  chert  rock  which  forms  hills  200  to  400 
feet  above  the  underlying  limestone.  The  latter  forms  the 
base  of  the  hills  20  to  150  feet  above  the  bottom  of  the 
valleys.  The  springs  emerge  on  a  thin  bed  of  shale  which 
separates  the  two  rocks. 

springs  and  are  caused  by  the  groundwater  in  its  descent 
from  the  surface  meeting  a  bed  of  clay,  shale,  or  other 


60  PHYSICAL  GEOGRAPHY 

dense  rock  and  following  along  the  top  of  this  layer  until 
it  emerges  on  the  surface.     (Figs.  36  and  37.) 


Tig.  37.  Vertical  section  through  Eureka  Spring,  Ark.  Com- 
pare with  Fig.  36.  The  groundwater  percolates  through 
the  upper  layer  B  faster  than  it  can  penetrate  the  shale 
layer.  It  moves  along  the  top  of  the  shale  until  it  emerges 
at  the  surface  forming  springs  at  s,  s,  s,  s. 

Seepage. — Ground  water  generally  collects  into  little 
streams  on  the  top  of  the  impervious  layer  and  the  emerg- 
ence of  such  a  stream  at  the  surface  forms  a  spring.  Some- 
times, however,  the  water  flows  in  a  sheet  along  the  entire 
surface  of  the  dense  layer  and  then  instead  of  flowing  out 
in  streams,  it  seeps  or  trickles  out  along  the  line  of  outcrop 
of  the  layer  in  sufficient  quantities  to  keep  the  surface  wet, 
frequently  forming  a  swamp  or  bog  on  the  hillside.  This 
is  called  a  seepage  spring. 

Fissure  Springs.— Fissure  springs  consist  of  those  in 
which  the  water  in  its  underground  passage,  enters  a 
fissure  or  crack  leading  to  the  surface  through  which  the 
water  emerges  under  hydrostatic  pressure,  as  in  an  arte- 
sian well. 

62.  Mineral  Springs. — All  spring  water  contains  some 
mineral  matter  in  solution  but  certain  ones  known  as  min- 
eral springs  are  characterized  by  an  excessive  amount  of 
some  common  mineral  matter,  as  carbonate  of  lime,  car- 
bonate of  iron,  or  hydrogen  sulphide,  or  by  the  presence 
of  some  rare  compounds  like  those  of  lithium.  The  most 
common    mineral    springs    are    the    lime,    sulphur,    iron 


GROUNDWATER  AND  RIVERS  61 

(chalybeate),  magnesia,  carbonic  acid,  potash,  soda,  lithia, 
silica  springs.  Some  of  the  mineral  springs  are  hot  and 
others  are  cold. 

Some  have  a  wide  reputation  for  the  curative  proper- 
ties of  the  water,  for  the  benefit  of  which  people  travel 
long  distances.  In  some  places  the  waters  are  bottled 
and  shipped  to  distant  points.  What  mineral  springs  can 
you  name  in  New  York  State?  Your  own  State?  Make 
a  list  of  the  places  and  the  kind  of  springs.  The  springs 
shown  in  fig.  36  are  widely  known  mineral  springs. 

63.  Hot  springs  are  those  in  which  the  water  has  a  high 
temperature,  sometimes  at  or  near  the  boiling  point.  Such 
springs  occur  in  the  region  of  active  or  extinct  volcanoes,  where 
the  rocks  have  not  yet  cooled  from  the  former  highly  heated  con- 
dition. The  circulating  groundwaters  coming  in  contact  with 
these  heated  rocks  below  the  surface  are  warmed  and  emerge 
in  the  spring  as  hot  water. 

In  some  places  where  hot  springs  are  remote  from  any  vol- 
canic rocks,  they  may  be  caused  (1)  by  intrusive  molten  rocks 
which  have  not  reached  the  surface;  or  (2)  by  heat  produced 
by  friction  in  the  bending  and  fracturing  of  the  rocks  in  the  fold- 
ing of  mountains;  or  (3)  by  chemical  action  going  on  in  the 
rocks  through  which  the  water  is  passing.  Hot  springs  are  found 
in  mountainous  countries,  in  Arkansas,  Virginia,  South  Dakota, 
and  many  of  the  Rocky  Mountain  and  Pacific  States. 

64.  Geysers. — Geysers  are  boiling  springs  that  erupt 
intermittently.  The  water  is  thrown  out  periodically, 
sometimes  to  a  height  of  several  hundred  feet.  The 
eruptions  take  place  in  some  geysers  at  quite  regular  in- 
tervals, while  in  others  the  intervals  are  very  irregular, 
sometimes  several  hours  or  several  days. 

The  water  in  all  geysers  contains  alkali  (soda  or  potash) 
in  solution,  which  in  turn  dissolves  silica  in  the  deeper 
portions  of  the  circulation.  When  the  heated  silica-bear- 
ing waters  approach  the  surface  the  decrease  in  pressure 
and  loss  of  temperature  causes  some  of  the  silica  to  be 


62 


PHYSICAL  GEOGRAPHY 


deposited  along  the  sides  of  the  opening,  making  a  smooth 
but  crooked  and  irregular  tube  through  which  the  water 
finds  its  way  to  the  surface. 

The  eruption  of  the  geyser  is  caused  by  the  high  temperature 
in  the  deeper  portions  of  the  tube  which  causes  the  water  to  be 
heated  above  the  boiling  point.  A  portion  of  it  finally  changes 
to  steam,  the  expansion  of  which  lifts  the  plug  of  colder  water 
in  the  upper  part  of  the  tube,  causing  it  to  overflow  and  thus  re- 
lease the  pressure  on  the  water  towards  the  bottom.     This  re- 


FiG.  :i8.  Old  Faithful  geyser  in  Yellowstone  National  Park.  The 
white  mound  is  composed  of  silica  deposited  from  the  waters 
of   the   geyser.      (U.    G.    Cornell.) 

lease  of  pressure  permits  a  large  volume  of  steam  to  form  sud- 
denly, and  forcibly  expel  all  the  water  from  the  tube.  The 
water  partially  cooled  in  the  air  runs  back,  fills  up  the  tube  and 
stands  until  the  bottom  is  again  heated  above  the  critical  point, 
that  is,  the  point  where  the  water  will  change  to  steam  under  the 
existing  pressure,  when  another  eruption  takes  place.     The  crit- 


GROUNDWATER  AND  RIVERS  63 

ical  point  where  water  changes  to  steam  at  sea  level  on  the  sur- 
face of  the  earth  is  212  degrees  F.,  but  deep  beneath  the  surface 
under  the  increased  pressure  it  may  reach  a  temperature  of  250 
degrees  or  more  before  it  forms  steam.  Another  possible  explan- 
ation for  some  of  the  geysers  is  that  steam  accumulates  in  an 
underground  cavity  until  it  has  sufficient  force  to  overcome  the 
resistance  of  the  column  of  water  when  it  violently  expels  the 
water  from  the  opening. 

The  constant  loss  of  heat  from  the  eruptions  of  hot  water 
causes  a  decrease  in  the  activity  of  the  geyser  which  in  time  be- 
comes a  hot  spring  and  finally  a  cold  spring.  In  the  Yellowstone 
Park  there  are  about  3,000  openings,  some  of  which  are  geysers 
but  the  majority  of  them  are  now  springs,  some  hot  and  others 
cold.  A  decrease  in  the  activity  of  some  of  the  geysers  has 
been  noted  in  the  past  few  decades.  Even  "Old  Faithful"  that 
formerly  erupted  regularly  every  hour  is  now  becoming  irregular 
with  sometimes  an  interval  of  an  hour  and  a  half  between  erup- 
tions. The  decrease  in  activity  of  some  of  the  geysers  is  balanced 
in  part  at  least  by  increased  activity  in  others. 

At  present  there  are  four  geyser  localities  known  in  the 
world:  one  in  the  Yellowstone  National  Park  in  Wyoming,  one 
in  Iceland,  one  in  New  Zealand,  and  one  in  South  America  near 
the  headwaters  of  the  Amazon. 

RIVERS 

Part  of  the  rain  that  falls  does  not  sink  into  the 
gronnd  at  all  bnt  runs  directly  into  depressions  and 
through  these  to  the  sea.  It  mingles  on  the  way  with  the 
water  from  springs,  seepage,  and  the  melting  snows, 
which  all  together  form  the  brooks,  creeks,  and  rivers 
which  fill  the  lake  basins,  and,  running  into  the  sea,  re- 
place the  loss  that  results  from  evaporation.  The  moisture 
evaporated  from  the  ocean  is  carried  as  vapor  through  the 
air,  falls  as  rain  or  snow  upon  the  land,  and  is  carried 
back  to  the  ocean  through  the  rivers.  In  this  great  cir- 
culating system,  the  rivers  are  of  special  interest  to  the 
geographer,  because  they  have  more  to  do  with  modify- 
ing the  surface  of  the  earth,  and  in  sculpturing  the  beau- 


64  PHYSICAL  GEOGRAPHY 

tiful  and  varied  features  of  the  landscape  than  any  of  the 
other  parts  of  the  system. 

65.  Origfin  of  the  River  Valley.— River  valleys  may 
begin  and  grow  in  one  or  both  of  two  ways:  (1)  The  rain 
that  falls  on  the  border  of  a  new  land  mass  forms  gullies, 
some  of  which  deepen  and  lengthen  and  widen  until  they 
form  river  valleys.  (2)  The  rain  that  falls  on  a  new 
land  area  runs  into  existing  depressions  until  they  are 
filled,  each  overflowing  into  the  next  lower  one,  and  from 
the  lowest  into  the  sea.  The  depressions  when  first  filled 
with  water  are  lakes,  which  in  time  are  filled  up  with 
sediment.  The  streams  cut  gorges  and  valleys  between 
the  lake  basins  and  finally  through  the  filled  basins  until 
there  is  a  continuous  channel  for  the  river  from  the  inner- 
most rainfall  to  the  sea.  Probably  in  all  river  systems 
there  is  a  combination  of  these  two  methods.  Since  the 
first  method  is  the  simplest  it  will  be  described,  and  the 
reader  can  apply  the  same  principles  to  the  second. 

66.  Gullies. — The  rain  on  the  hillside  washes  away 
the  softer  material  first  and  forms  a  little  depression  or 
gully.  The  depression  gathers  more  water  and  so  con- 
tinues to  wear  faster  than  the  land  on  either  side.  After 
the  gully  is  once  started,  the  frost  and  wind  and  the  other 
weathering  agents  (what  are  the  others?)  assist  the  rain 
in  loosening  and  moving  away  the  material  at  the  sides 
and  at  the  head  of  the  gully.  Refer  to  sections  71,  212  and 
213  for  description  of  weathering  and  disintegration  of 
rocks. 

The  material  so  loosened  is  carried  by  the  rain,  assisted 
by  gravity,  to  the  bottom  of  the  gully  where  it  is  swept 
along  first  by  the  temporary  stream,  and  later  by  the  per- 
manent stream,  and,  as  it  grinds  against  the  sides  and 
bottom  of  the  channel,  it  corrades  or  wears  away  frag- 
ments of  the  rock  and  thus  lowers  the  bottom  of  the  gully. 


GROUNDWATER  AND  RIVERS 


65 


^jS^ 


'£v;Tr*; 


Fig.  39.  Gullies  and  valleys  in  youtli  in  soft  material  in  an 
arid  region.  Compare  with  Fig.  64.  Though  the  annual 
rainfall  is  light  it  is  concentrated  in  heavy  showers.  Bates' 
Hole,   Wyoming.      (U.   G.   Cornell.) 


Fig.  40.  Telephoto  view  of  gullies  in  the  Bad  Lands,  South 
Dakota.  Gullies  in  a  dry  climate.  Compare  with  Figs.  39 
and    42. 


66 


PHYSICAL  GEOGRAPHY 


As  this  process  goes  on,  the  bottom  of  the  gully  is  finally 
cut  below  the  water  table  into  the  zone  of  perpetual 
groundwater  and  then,  and  not  till  then,  does  the  valley 
gain  a  permanent  stream.     The  erosion  then  goes  on  con- 


JFiG.  41,  Toad  Stool  Park,  Adelia,  Nebraska.  Copyright  1898  by  E.  H. 
Barbour.  Not  every  gully  becomes  a  river  valley.  Observe  the 
great  number  of  gullies  in  proportion  to  the  stream  valleys  in  the 
hills  in  the  background, 

tinuously  throughout  the  year,  and  is  supported  by  the 
springs  and  seepage  that  emerge  at  the  sides  of  the  valley 
and  keep  up  the  water  supply  during  the  dry  season. 

The  head  of  the  gully  or  valley  may  extend  back  into 
the  land  area  until  it  reaches  a  permanent  divide  where 
the  streams  flow  in  another  direction.  As  the  valley 
lengthens,  tributaries  develop  along  the  side  similar  to 
the  original  gully,  and  tributaries  to  these  in  turn  until 
the  area  is  covered  with  a  great  system  consisting  of  the 
main  stream  with  all  its  tributaries. 


GROUNDWATER  AND  RIVERS  67 


Pig.  42.  Gullies  in  hard  volcanic  rock  on  the  side  of  Mt.  Potosi,  Ouray  Co., 
Col.  Compare  witli  FiGS.  39  and  64.  The  rocks  shown  in  this  view  are 
all  hard  volcanic  lavas. 

It  must  not  be  inferred  that  every  gully  becomes  a 
river  valley.  To  one  such  there  are  thousands  that  are  never 
anything  but  gullies,  and  possibly  not  large  ones  at  that. 

67.  Definition. — A  river  is  a  stream  of  water  together 
with  the  rock  waste  which  it  carries.  It  has  its  source  in 
the  springs,  seepage,  rainfall,  and  snowfall  in  and  around 
the  upper  end  of  its  valley.  Not  infrequently  a  lake  is 
the  source  of  a  river,  but  generally  it  is  only  part  of  a 
river,  in  a  wider  and  expanded  portion  of  the  valley;  and 
the  stream  or  streams  that  flow  into  the  lake  are  but  the 
headwaters  of  the  river  that  flows  out  of  it.  Some  rivers 
have  their  sources  in  melting  glaciers.  Small  streams  are 
commonly  called  creeks,  brooks  or  rivulets  but  the  distinc- 
tion is  seldom  made  in  geography  because  it  is  merely  one 


68 


PHYSICAL  GEOGRAPHY 


of  size  and  what  is  called  a  creek  in  one  locality  may  be 
larger  than  one  called  a  river  elsewhere. 

The  land  over  which  the  stream  flows  is  the  bed  and  that  im- 
mediately bordering  the  stream  confining  it  to  the  bed  forms  the 
baiiks.  The  mouth  of  the  river  is  the  place  where  it  flows  into 
the  sea,  a  lake,  or  another  river.  A  river  basin  is  all  the  land 
drained  by  the  river  and  its  tributaries.  A  river  system  includes 
the  main  river  and  all  its  tributaries.  A  divide  is  the  parting  be- 
tween the  surface  waters  of  two  valleys  or  basins.  It  may  be  a 
sharp  ridge  in  mature  topography,  or  a  broad  flat  or  plateau  in 
young  topography,  or  it  may  rarely  be  a  body  of  water.  The 
divide  between  the  Amazon  and  Orinoco  rivers  is  so  low  in  one 
place  that  it  is  possible  to  cross  it  in  a  boat;  in  fact,  it  shifts 
back  and  forth  over  a  considerable  length  of  water  depending  on 
the  local  rainfall  and  the  height  of  the  water  in  the  two  rivers. 
(Trace  out  and  describe  the  divides  on  the  Charleston  and  Ottawa 
or  Fargo  sheets  of  the  Topographic  Atlas.  The  first  is  an  ex- 
ample of  mature  topography  and  the  other  two  of  youthful  topog- 
raphy.    See  Sec.  85) 


LAK 

ETEA 

R  OF! 

HE  CI 

OUDS 

-^ 

^ 

/ 

o 

z 

/ 

^ 

i 

>> 

//i 

dsaa. 

5_ 

o 

^ 

-"     Mohair/: 

Fia.  43.  Profile  of  the  Hudson-Mohawk  River.  Observe  the 
contrast  between  the  upper  and  lower  Hudson  and  between 
the  upper  Hudson  and  Mohawk.  Vertical  scale  2000  feet 
to  one  inch.  Horizontal  scale  100  miles  to  one  inch. — 
(U.    S.    Geol.    Survey.) 


GROUNDWATER  AND  RIVERS 


69 


68.  Profile  of  a  River.— The  slope  of  a  river  channel  or  the 
angle  of  the  inclination  to  the  horizontal  is  one  of  the  chief 
factors  in  determining  the  velocity  of  a  river;  the  steeper  the 
slope  the  more  rapid  the  current.  A  line  representing  the  slope 
of  a  river  channel  from  the  headwaters  to  the  mouth  of  a  river 
is  called  the  profile.  In  general  the  slope  is  steeper  near  the 
headwaters  of  the  river  and  decreases  towards  the  mouth.  Com- 
pare the  profiles  of  some  of  the  principal  rivers  of  the  United 
States  and  draw  the  profile  of  a  river  from  furnished  data.  (See 
Bulletin  44  of  the  Water  Supply  and  Irrigation,  Papers  published 
by  the  U.  S.  Geological  Survey.) 


Pig.  44.  East  branch  of  Limestone  Creek,  Manlius,  N.  Y.,  show- 
ing reach  or  pool  in  foreground  and  rapids  in  the  back- 
ground. Very  little,  almost  no  erosion  is  going  on  at  the 
reach.  On  the  rapids  the  rocks  are  being  ground  to  pieces 
and  the  finer  portions   carried  to  a  lower  level. 


69.  Reaches  and  Rapids.— A  river  is  rarely  if  ever 
of  uniform  slope  from  the  mouth  to  the  head,  but  gener- 
ally consists  of  a  series  of  alternating  pools  or  stretches  of 
quiet  water  called  reaches  separated  from  each  other  by 
ripples  or  rapids  where  the  current  flows  swiftly.  Por- 
tions of  the  pools  are  generally  being  filled  with  sand  and 
mud,  and  the  rapids  are  cutting  the  channel  deeper  and 


70 


PHYSICAL  GEOGRAPHY 


Fig.  45.  West  branch  of  Limestone  Creek, 
Manlius,  N.  Y.,  showing  rapids  in  the  fore- 
ground. At  this  point  the  stream,  owing 
to  its  curvature,  is  cutting  sideways  and 
widening  its  valley  at  the  reach.  Notice 
the    undercutting    of    the    bank    on    the    right. 


■r 

MmB^ 

1 — i 

Fig.  46.  Potomac  River,  Barnum,  Md.,  showing  rapids  at  low 
water  and  the  tools  which  the  stream  uses  in  degrading  its 
channel.  Most  of  the  work  done  by  the  stream  is  accom- 
plished during  the  high  water  stage.  (Maryland  Geological 
Survey. ) 


GROUNDWATER  AND  RIVERS  71 

slowly  receding  towards  the  headwaters.  In  the  pools  or 
flats  the  river  is  aggrading  its  channel,  that  is,  depositing 
sediment  and  raising  its  bed;  on  the  rapids  the  stream  is 
degrading  its  channel,  that  is,  cutting  it  deeper.  Some 
portions  of  the  river  course  are  neither  aggrading  nor 
degrading  but  are  graded.  As  a  stream  advances  in  age 
the  graded  portions  tend  to  increase  in  length  and  when 
they  are  all  connected  and  all  rapids  have  disappeared, 
the  stream  is  completely  graded  and  has  reached  base 
level.     (Study  figs.  44,  45  and  46.) 


Fig.  47.  Streams  flowing  into  a  lake  form  deltas 
and  deposit  sediment  until  the  lake  basin  is 
filled  to  the  level  of  the  outlet.  Compare  with 
Fig.  48. 

70.  Base  Level. — The  base  level  is  reached  by  a  river 
when  the  rapids  disappear  and  as  a  stream  of  clear  water 
it  flows  with  a  slow  motion,  neither  eroding  nor  deposit- 
ing material,  over  a  plain  having  a  gentle,  uniform  incli- 
nation from  the  headwaters  to  the  mouth.     It  is  the  low- 


72 


PHYSICAL  GEOGRAPHY 


est  level  to  which  the  river  mechanically  erodes  the  land 
over  which  it  is  flowing.  (Some  writers  define  the  term 
hose  level  as  the  level  of  the  sea  or  other  body  of  water 
into  which  the  river  flows.  For  a  discussion  of  the  mean- 
ing of  the  term  see  Davis,  Journal  of  Geology,  Vol.  x,  p. 

77.) 

71.  Work  of  Rivers.— The  work  of  a  river  consists  in  dis- 
secting the  upland  areas  and  carrying  the  material  along  with 
the  excess  rainfall  to  the  lowlands  and  finally  to  the  sea.     In  this 


Fig.  48.  After  the  lake  is  filled,  the  stream  from 
the  outlet  is  supplied  with  sediment,  degrades 
its  channel,  and  then  picks  up  and  carries 
onward  the  sediment  formerly  deposited  in  the 
lake  basin.     Compare  with  FlG.  47. 


work  it  is  assisted  by  the  weathering  agents,  such  as  wind,  frost, 
heat  of  the  sun,  gravity,  chemical  action,  and  animal  and  plant 
life,  which  cause  the  rocks  to  disintegrate  and  crumble  into 
fragments.  The  rain  washes  the  loose  materials  into  the  stream 
channels,  and  the  current  carries  them  toward  the  sea.  The 
journey  is  not  a  continuous  one,  because  the  material  is  dropped 
and  picked  up  again  many  times  between  the  mountain  and  the 


GROUNDWATER  AND  RIVERS  73 

sea.  Thus,  when  a  river  flows  into  a  pond  or  lake,  nearly  all  the 
sediment  is  deposited  until  the  lake  is  completely  filled,  after 
which  the  river,  under  new  conditions,  picks  up  the  materials  and 
carries  them  on  to  a  lower  level.  Most  of  the  transportation  is 
done  during  flood  season,  but  the  disintegration  goes  on  contin- 
uously throughout  the  year.     (See  figs.  47  and  48.) 

72.  Corrasion.— The  material  carried  by  the  stream, 
the  sand,  mud,  and  gravel,  corrades  or  grinds  away  the 
rocks  in  the  channel.     The  sand  and  pebbles  are  the  tools 


Fig.  49.  Pot  hole  formed  in  rock  by  the  grinding  action  of  the 
pebbles  and  sand  in  the  eddying  waters  on  the  rapids.  (U.  S. 
Geol.   Survey. ) 

that  do  the  grinding,  while  the  water  acts  as  a  carrier. 
In  the  rapids,  eddies  are  formed  and  pebbles  are  caught 
and  swirled  around  in  the  w^aters  until  a  circular  depres- 
sion or  pot-hole  is  formed  which  may  be  from  a  few  inches 
to  many  feet  in  depth  and  diameter.  Pot-holes  are  also 
formed  underneath  glaciers  where  a  surface  stream  de- 
scends through  a  crevasse  or  moulin.  (See  Sec.  126,  fig. 
116).     Deep,  narrow  gorges  like  Ausable  Chasm  and  Wat- 


74  PHYSICAL  GEOGRAPHY 

kins  Glen  indicate  rapid  corrasion  or  down-cutting  by  the 
stream. 

73.  Transportation.— Rivers  transport  materials  in 
several  different  ways:  (1)  Floating  on  the  surface. 
Great  quantities  of  vegetable  material  are  carried  in  this 
way  and  at  times  limited  quantities  of  earthy  material. 
Where  the  water  rises  gradually  over  a  dry  sand  deposit, 
little  cakes  and  patches  of  the  dry  sand  are  floated  away 
on  top  often  a  long  distance  before  they  are  disturbed 
and  sink.  Place  a  sewing  needle  gently  on  the  surface  of 
a  cup  of  water  and  it  will  float  in  the  same  way.  When 
the  river  undercuts  its  banks  and  a  part  of  the  forest  or 
vegetable  covering  slides  into  it,  the  earthy  material 
clinging  to  the  roots  is  carried  away  with  the  floating 
trees  and  grass.  In  cold  climates  the  blocks  of  ice  borne 
away  on  the  spring  floods  frequently  carry  fragments  of 
rock  and  earth  long  distances  down  the  stream  before  the 
ice  melts  and  drops  them  in  the  channel.  (2)  Great 
quantities  of  mud  and  fine  sand  are  carried  along  sus- 
pended in  the  water  borne  up  by  the  many  upward  whirls 
in  the  current.  (3)  Boulders,  pebbles,  and  sand  are 
rolled  and  pushed  along  the  bottom,  and  hence  are  being 
continually  worn  rounder,  smoother  and  smaller.  The 
larger  boulders  are  moved  in  the  more  rapid  portions  of 
the  stream  as  in  the  mountain  torrents,  while  through 
the  flood  plain  in  the  lower  portion  of  the  river,  the 
burden  is  mostly  sand  and  mud.  It  is  surprising  how 
great  a  quantity  of  sand  is  being  moved  along  the  bottom 
of  the  lower  courses  of  a  great  river  like  the  Mississippi 
or  the  Missouri.  (4)  Besides  the  above  there  is  the  in- 
visible load  which  the  river  carries  in  solution,  consist- 
ing of  compounds  of  lime,  iron,  magnesia,  soda,  potash 
and  minute  quantities  of  other  materials  dissolved  from 
the  rocks.     This  part  of  the  work  goes  on  all  the  year. 


GROUNDWATER  AND  RIVERS  75 

even  during  the  low  water  stage  in  the  dry  season  when 
the  waters  are  free  from  sediment. 

74.  Transporting  Power  of  Streams.— The  carrying 
power  of  streams  varies  greatly  with  the  velocity  of  the 
water,  the  velocity  being  frequently  a  factor  of  the  vol- 
ume. In  the  Johnstown,  Pa.,  flood  the  great  volume  of 
water  from  the  broken  reservoir  moved  down  the  same 
slope  as  that  over  which  the  Conemaugh  river  flows  at  all 
times.  Yet  the  force  of  the  flood,  due  to  the  increased 
volume  of  water,  was  sufficient  to  twist  railroad  irons,  move 
freight  cars,  and  even  railway  locomotives,  and  cause  enor- 
mous destruction  of  property.  The  increase  in  the  carry- 
ing power  is  in  a  much  greater  ratio  than  the  increase  in 
velocity,  hence  the  great  destruction  wrought  by  streams  in 
flood  time.  The  carrying  power  increases  as  the  sixth 
power  of  the  velocity.  That  is,  if  the  velocity  is  increased 
10  times  the  carrying  power  is  raised  1,000,000  times. 

The  transporting  power  of  streams  having  different  velocities 
is  shown  by  the  following: 

3  inches  per  second  will  just  move  fine  clay. 
6  inches  per  second  will  move  fine  sand. 

12  inches  per  second  will  move  fine  gravel. 

24  inches  per  second  will  move  pebbles  an  inch  in  diameter. 

36  inches  per  second  will  move  pebbles  as  large  as  an  egg. 

10  miles  per  hour  will  move  masses  of  1^/^  tons. 

20  miles  per  hour  will  move  masses  of  100  tons. 

The  load  carried  by  a  stream  is  often  a  small  fraction  of  the 
carrying  capacity  of  the  stream.  If  the  water  is  flowing  over 
hard  rock  it  may  be  unable  to  pick  up  any  load.  It  is  not  always 
the  rapid  current  that  is  carrying  the  greatest  load;  although  it 
has  the  capacity  it  may  not  have  the  load  to  carry. 

75.  How  the  Energy  of  the  Stream  is  Expended.— 

Part  of  the  potential  energy  of  the  stream  is  used  up  in 
transporting  its  load,  and  part  in  corrading  its  channel. 
If  the  stream  is  full  loaded,  that  is,  has  all  the  material  it 


76 


PHYSICAL  GEOGRAPHY 


Fig.  50.  An  underloaded  stream  which  is  corrading  its  channel 
in  granite.  View  in  the  canyon  of  the  North  Platte  River. 
(U.  G.  Cornell.) 


Fia.  51.  An  overloaded  stream,  Tncninpai^'hre  Creek,  Ouray, 
Col.  Above  this  point  the  stream  flows  through  narrow 
canyons,  similar  to  that  in  Fia.  50,  carrying  much  sediment 
which  is  deposited  in  this  broader  part  of  the  valley  below 
the  canyons.  Along  with  the  bowlders,  gravel  and  sand 
there   is  considerable   driftwood  deposited. 


GROUNDWATER  AND  RIVERS 


77 


can  carry,  it  will  not  do  any  eroding.  If  it  is  overloaded 
it  will  deposit  part  of  the  load;  if  it  is  underloaded,  the 
excess  energy  will  be  expended  in  deepening  and  widening 


Fig.  52.  An  overloaded  stream,  Uncompaghre  Creek  above  Sneffels,  Col. 
The  same  stream  as  in  Fig.  51  but  nine  miles  nearer  the  source. 
Note  the  narrower  valley,  the  steeper  slope  in  the  channel  and  the 
coarser  material  deposited  in  the  channel.  The  white  spot  near  the 
middle   of  the  picture   is  a  cataract. 

the  channel  by  corrasion.  The  Missouri  river  is  over- 
loaded across  the  plains  and  underloaded  on  most  of  its 
course  through  the  Rocky  Mountains.  Hence  it  is  deep- 
ening its  channel  through  the  mountains  and  building  it 
up  across  the  plains.  (Explain  the  different  ways  in  which 
a  stream  may  become  overloaded.  See  figs.  51,  52  and  53). 
76.  Deposits  Made  by  the  River.— Probably  the  lar- 
gest deposits  made  by  the  river  are  those  made  on  the 
flood  plain  and  in  the  delta,  but  before  the  sand  and  mud 


78 


PHYSICAL  GEOGRAPHY 


reach  either  of  these  stopping  places  they  may  have  been 
dropped  and  picked  up  many  times  and  had  many  rough 
tumbles  and  a  varied  experience  in  the  upper  courses  of 
the  streams. 


Fig.  53.  An  overloaded  stream,  Taughannock  Creek,  N.  Y. 
The  mass  of  material  in  the  channel  has  been  swept  by 
the  stream  in  flood  season  from  the  narrow  gorge  in  the 
hills  in   the   background.      (E.    R.    Smith.) 

The  bulk  of  the  material  moved  by  a  river  is  carried  in  flood 
time  because  of  the  increased  volume  and  velocity  of  the  water, 
but  it  is  frequently  moved  only  a  short  distance,  to  be  dropped 
until  moved  again  by  a  subsequent  flood.  Sometimes  a  stream 
may  be  so  overloaded  that  it  will  deposit  part  of  the  load  and 
pick  it  up  again,  when  it  has  no  load,  even  at  low  water. 

77.  Flood  Plains.— All  streams  as  they  approach  old 
age  develop  more  or  less  extensive  plains  bordering  the 
channel,— plains  so  low  that  they  are  covered  with  water 
during  flood  season  in  the  river,  hence  the  name  flood 
plain.  The  water  covering  the  flood  plain  is  the  overflow 
from  the  channel  and  has  lost  much  of  its  former  velocity, 
because,  first  the  water  is  shallower  than  in  the  channel, 
and,  second,  the  vegetation  acts  as  a  check  on  the  velocity. 


GROUNDWATER  AND  RIVERS  79 

Hence  much  of  the  sediment— the  sand  and  mud— is  de- 
posited, forming  the  rich  alluvial  soil  that  makes  the  flood 
plains  such  rich  farming  regions. 


Fig.  54.  Flood  plain  of  stream  flowing  into  Chesapeake  Bay, 
Calvert  Co.,  Md.  Rivers  generally  have  a  meandering 
course  on    a  flood  plain.      (Maryland  Geological   Survey.) 

The  flood  plains  are  very  large,  covering  many  thou- 
sands of  square  miles  on  the  lower  courses  of  old  and  large 
rivers  like  the  Mississippi  and  the  Nile.  In  general  the 
flood  plain  narrows  in  ascending  the  river.  Frequently 
in  the  middle  and  upper  courses  of  the  streams  there  is  a 
narrow  flood  plain  on  one  side  of  the  stream  while  the 
other  flows  against  the  bordering  cliff. 

Why  are  there  no  large  flood  plains  on  the  Hudson  and  the 
Niagara  rivers?  Do  you  know  of  any  large  flood  plains  on  any  of 
the  rivers  in  New  York?  (See  flgs.  54,  55,  and  57  and  map  of 
the  lower  Mississippi  River.) 

78.  Meanders. — When  a  river  has  graded  a  portion 
of  its  course  and  formed  a  flood  plain,  it  ceases  to  corrade 
the  bottom  of  its  channel  at  that  place  but  may  continue 
to  cut  at  the  sides  or  banks.  Where  the  current  is  de- 
flected to  one  side  it  cuts  away  the  bank  at  that  point,  pro- 


80  PHYSICAL  GEOGRAPHY 

ducing  a  curve  which  deflects  the  current  across  to  the 
opposite  bank  below,  where  another  curve  is  formed.     In 


Pig.  55.  Meandering  stream — Coal  Creek  on  the  Laramie  plains, 
Wyoming.  This  stream  is  utilized  to  transport  the  logs  from  the 
forests  in  the  adjoining  mountains  to  the  mills  far  out  on  the  plains. 
U.    G.   Cornell.) 


Fig.  56.  Meanders,  M,  M',  M",  R,  R,  R,  former  course  of  the 
river.  C  beginning  of  a  cut-off.  D  is  a  cut-off.  The  ends 
of  the  lagoon  LL  are  being  filled  with  sand  and  mud  at  a,  a. 

both  places  the  bank  is  worn  away  and  the  stream  in  time 
becomes  quite  crooked  or  meandering.  While  the  stream 
is  cutting  the   outside   of  the   meander  curve,   sand   and 


GROUNDWATER  AND  RIVERS  81 

gravel  are  being  deposited  on  the  inside  of  the  curve 
(why?  study  the  diagrams)  and  the  whole  channel  is 
gradually  shifted  into  or  even  across  its  flood  plain.     The 


Fig.  57.  Map  of  portion  of  the  Mississippi  River,  showing 
meanders  and  cut-offs.  The  lakes  are  ox-bow  lakes  and 
portions  of  the  abandoned  channel.  Note  the  sand  de- 
posits a,  a,  a,  on  the  inner  bank  at  many  of  the  curves. — 
(After   the    Miss.    River    Com.) 

meander  curve  once  started  continues  to  increase  its  cur- 
vature until  the  stream  cuts  across  the  neck  between  two 
approaching  curves  and  thus  straightens  that  portion  of 
the  channel.  The  cut-off  is  gradually  silted  up  at  the 
ends,  forming  first  a  lagoon  and  later  an  ox-bow  lake. 
These  ox-bow  cut-offs,  so  common  on  the  flood  plains  of 
6 


82  PHYSICAL  GEOGRAPHY 

large  rivers,  may  frequently  be  observed  on  small  brooks 
where  they  are  following  a  winding  course  through  a 
meadow.  Study  the  Mississippi  River  maps  and  figs.  55, 
56  and  57. 

When  the  meander  curve  reaches  the  outer  limit  of  the  flood- 
plain  of  the  river,  it  begins  to  undercut  the  river  bluff  and  thus 
widen  the  valley.  Study  meander  curves  on  nearby  streams; 
where  possible  observe  them  at  intervals  of  a  few  months  or 
years.  Suggestion  to  the  teacher — take  photographs  each  year 
for  comparison.     (See  fig.  45.) 

79.  The  natural  levee  is  formed  on  the  flood  plain 
on  the  immediate  bank  of  the  stream.  In  flood  season  the 
river  overflows  its  banks  and  spreads  out  in  a  thin  sheet 
of  water,  moving  slowly  down  the  valley  over  the  level 
area,  but  the  water  in  the  channel  moves  much  more 
rapidly  than  that  on  the  flood  plain  because  it  is  much 
deeper.  At  the  contact  of  the  swiftly  moving  water  of 
the  channel  and  the  slower  moving  water  of  the  flood 
plain,  there  is  a  deposition  of  sand  and  mud  due  to  the 
check  in  the  velocity.  The  increase  in  size  of  this  em- 
bankment is  aided  by  the  dense  growth  of  willows,  alders, 
and  other  bushes  along  the  bank  which  catch  the  drift  and 
add  it  to  the  bank  as  well  as  aid  in  checking  the  current 
and  adding  to  the  deposit  of  the  sediment. 

80.  Artificial  Levees.— Man  attempts  to  improve  on  nature's 
methods  by  adding  material  to  the  top  of  the  natural  levees,  pro- 
ducing artificial  ones  in  the  endeavor  to  keep  the  river  in  its 
channel  and  prevent  it  from  overflowing  the  flood  plain.  As  the 
levee  is  built  up,  the  river  deposits  material  on  the  bottom  of  the 
channel,  making  it  necessary  to  keep  adding  to  the  top  of  the 
levee  until  it  is  sometimes  built  up  many  feet  above  the  border- 
ing area,  so  that  steamboats  in  the  river  are  sometimes  above 
the  level  of  the  neighboring  farms.  A  break  in  such  a  levee 
(called  a  crevasse)  often  proves  to  be  very  destructive  to  the 
bordering  flood  plain.  Sometimes  the  river  even  leaves  the  exist- 
ing channel  permanently  and  forms  a  new  channel  elsewhere  to 


GROUNDWATER  AND  RIVERS 


83 


repeat  the  process,  and  in  this  way  by  a  repetition  of  such  changes 
finally  raise  the  level  of  the  entire  plain. 


^ 

"■    --%- 

^l^^^^y^ 

^ 

[ 

"-  ''"^*i 

it 

^^^^^^^  -*'- 

-  -^^"^^^^^f,,,^ 

-^^^i^^^—r.  -  ';■ 

,.„-— ^-w-5| 

Fig.  57a.     Strengthening  a  levee  on  the  Mississippi  River  at  Lagrange,   Miss., 
during  the  flood  season.      (National   Geog,    Mag.,   Oct.,    1907.) 

81.  River  Swamp.— The  river  swamp  generally  oc- 
cupies the  outer  borders  of  the  flood  plain  next  to  the 
river  bluffs,  because  the  building  up  of  the  natural  levee 
along  the  banks  of  the  channel  causes  the  plain  to  slope 
from  the  channel  towards  the  bluff  and  hence  causes  a 


5       c;  £ 


Fig.  5S.  Cross  section  of  a  river  valley — showing  the  flood 
plain,  river  swamp,  natural  levee  (the  small  elevation 
on  each  side  of  the  river  channel),  the  position  of  the 
water  table  (WP).  Liable  to  be  springs  or  seepage  and 
hillside   bogs    at    S    S. 

part  of  the  rainfall  on  the  flood  plain  to  drain  away  from 
the    stream    instead    of    toward    it.     Likewise    the    water 


84  PHYSICAL  GEOGRAPHY 

from  the  bluffs,  unable  to  find  its  way  across  the  natural 
levee,  accumulates  in  the  depressions  along  the  base  of 
the  bluff  and  aids  in  forming  the  river  swamp. 

82.  Levee  Lakes.— The  upbuilding  of  the  flood  plain  along 
the  channel  banks  sometimes  forms  a  dam  across  the  mouth  of 
a  tributary,  and  thus  produces  a  lake  on  the  tributary,  while  in 
other  places,  instead  of  forming  a  lake  the  tributary  flows  for 
many  miles  parallel  with  the  main  stream  between  the  levee  and 
the  bluffs  until  it  finds  a  place  where  it  can  penetrate  the  levee 
bank  into  the  main  stream.  The  above  points  should  be  studied 
on  detailed  maps  of  flood  plains  such  as  the  Donaldsonville  and 
Point  a  la  Hache,  La.,  contour  sheets  of  the  topographic  atlas 
and  the  sheets  of  the  Mississippi  River  Survey,  and  the  streams 
near  the  schoolhouse.     (See  fig.  221.) 

83.  Deltas. — A  river  flowing  into  a  body  of  still  water 
as  a  lake  or  the  ocean,  w^here  there  are  no  strong  tides, 
deposits  all  of  its  load  of  sediment,  building  up  an  accu- 
mulation called  a  delta.  There  is  generally  no  sharp  line 
of  separation  between  the  delta  and  the  flood  plain;  the 
latter  has  been  built  up  on  the  land  and  the  former  has 
been  formed  in  the  sea  or  lake.  The  river  divides  on  the 
delta  and  finds  its  way  into  the  sea  by  several,  sometimes" 
by  a  great  many  channels  called  distributaries.  The  head 
of  the  delta  is  frequently  located  where  the  first  distribu- 
tary leaves  the  main  channel. 


Pig.  59.  Cross  section  of  a  delta  showing  position 
of  the  beds.  A,  top-set  beds.  B,  fore-set  beds. 
C,   bottom-set  beds.      S,    sea   level. 

Deltas,  like  flood  plains,  have  a  fertile  soil  and  fre- 
quently support  a  dense  population.  Both  are  fertile 
because  they  are  composed  of  the  rich  surface  soil  from 
other  parts  of  the  basin.  It  contains  much  humus 
and  is  frequently  renewed. 


GROUNDWATER  AND  RIVERS 


85 


The  structure  of  the  delta  as  shown  by  the  diagram 
is  characteristic.  The  middle  portion  consists  of  mingled 
sand  and  mud  beds  formed  at  the  end  of  the  delta  where 
the  river  current  first  meets  the  still  water. 

At  this  point  a  larger  part  of  the  load  is  dropped  than 
at  any  other  and  the  material  comes  to  rest  in  inclined 
layers,  the  fore-set  beds,  dipping  towards  the  open  water. 


Fig.  60.  Walnut  canyon,  near  Flagstaff,  Ariz.,  showing  delta 
structure  in  the  sandstone  of  the  canyon  walls.  Commonly 
known  as  cross  bedding  or  false  bedding.      (A.  E.  Hackett.) 


But  some  of  the  fine  materials  is  carried  out  into  deeper 
water  forming  the  mud  layers  that  sometimes  reach  con- 
siderable thickness  and  great  extent,  the  hottom-set  beds, 
which  form  the  submarine  delta. 

The  delta  of  the  Indus  River  has  built  up  the  submarine  por- 
tion nearly  to  the  sea  level  over  such  an  extensive  area  of  now 
shallow  sea  that  in  places  large  vessels  cannot  even  get  within 
sight  of  the  shore. 

Over  the  top  of  the  inclined  beds  is  a  deposit  of  horizontal 
beds,  the  top-set  beds,  made  by  the  river  in  flood  time  and  not 
essentially    different    from    the    flood-plain    deposits.    All    these 


S6  PHYSICAL  GEOGRAPHY 

structural  features  may  be  observed  in  the  mud  deposit  formed 
in  the  pool  on  the  roadside  by  a  summer  shower. 

84.  Alluvial  Fan.— An  alluvial  fan  is  somewhat  like 
a  delta.  It  is  formed  at  the  point  where  a  mountain  tor- 
rent or  stream  with  a  steep  slope  carrying  a  great  deal  of 


Fig.  61.  An  alluvial  fan  at  Ouray,  Col.  The  fragmental  ma- 
terial on  which  the  trees  are  growing  was  swept  out  of  the 
narrow  canyon  faintly  visible  near  the  right  of  the  view. 

sediment  flows  out  on  a  valley  floor  or  flood  plain  of  a 
larger  stream,  or  on  a  plain  of  any  kind.  The  velocity  of 
the  swift  current  is  checked  suddenly  and  the  load  nearly 
all  deposited  at  the  border  of  the  plain,  building  up  a 
fan-shaped  mass  over  which  the  stream  flows  in  shifting 
channels  in  wet  weather.  In  dry  weather  the  water  fre- 
quently disappears  from  the  surface  entirely,  flowing 
through  the  mass  as  groundwater.  It  differs  from  a  delta 
in  being  built  up  on  the  land  instead  of  in  the  water,  hence 
the  material  is  not  well  stratified,  but  consists  of  a  jumbled 
mass  of  coarse  and  fine  deposit  almost  devoid  of  any 
stratification.  The  delta  of  a  large  stream  rarely  con- 
tains material  coarser  than  sand  or  very  small  pebbles 


GROUNDWATER  AND  RIVERS 


87 


but  the  alluvial  fan  sometimes  contains  boulders  mingled 
with  pebbles,  sand  and  mud.  If  the  stream  forming  the 
fan  is  small  and  descends  a  very  steep  slope  the  deposit 
will  have  a  steep  surface  and  resembles  the  section  of  a 


Fig.  62.  Tahis  conos  in  the  Rocky  Mts.,  Col.  The  rock 
fragments  loosened  by  the  frost  and  other  weathering 
agencies  roll  down  slight  depressions  on  the  moun- 
tain side  and  accumulate  in  conical  mounds  at  the 
base. 

cone,  when  it  is  called  an  alluvial  cone.  Talus  cones  are 
similarly  formed  by  gravity  and  rainwash  at  the  base  of 
steep  mountain  or  hill  slopes.     (Compare  figs.  61  and  62.) 

85.  Life  History  of  a  River— The  Cycle  of  Erosion.— 
The  successive  changes  which  a  stream  undergoes  from 
the  time  it  starts  on  an  upland  area  until  the  upland  has 
been  reduced  to  a  lowland  constitutes  the  life  history  of 
the  stream.  It  has  a  beginning,  a  period  of  development, 
decline  and  disappearance.  It  is  customary  to  distinguish 
at  least  three  different  stages  in  the  cycle  as  youth,  matur- 
ity and  old  age. 

Youth  is  the  period  of  rapid  growth  in  the  beginning 
of  the  cycle.     Some  of  the  characteristic  features  of  this 


88 


PHYSICAL  GEOGRAPHY 


stage  are  narrow  V-shaped  valleys,  cataracts  and  rapids, 
lakes  and  swamps  on  the  upland  and  inter-stream  areas, 
few  tributaries,  and  broad  stretches  of  undrained  or  poor- 


FlG.  63,  In  the  canyon  of  the  North  Platte  River.  Stream 
in  youth  in  hard  rock  on  an  arid  plateau  area.  (U.  G. 
Cornell.) 

ly  drained  country  bordering  the  valley.  The  conditions, 
of  course,  are  different  on  a  stream  developing  on  a  plain 
from  the  one  on  the  plateau  or  mountain,  yet  the  youthful 
stage  of  each  can  be  recognized  from  its  advanced  or  ma- 
ture stage   by  some  or  all  of  the  above  features.     Fox 


GROUNDWATER  AND  RIVERS 


89 


.2lmt^.i^,                     mm-  liii 

■^ 

"  -r^ 

Fig.  64.  Valley  in  youth  in  soft  material  formed  by  rain  wash  in  a 
humid  region,  Calvert  Co.,  Md.  Compare  with  FiGS.  63,  39,  40, 
and   42.      (Maryland   Geological    Survey.) 

River  on  the  Ottawa,  111.,  topographic  atlas  sheet  is  a  type 
of  topographic  youth.     (See  figs.  63  and  64.) 

Maturity  of  the  streams  is  characterized  by  the  ab- 
sence of  lakes  and  swamps,  which  have  been  filled  or 
drained,  the  absence  of  cataracts,  decrease  in  the  num- 
ber of  rapids,  increase  in  the  number  of  tributaries,  and 
complete  dissection  of  the  inter-stream  areas.  The  di- 
vides are  sharp  ridges.  There  is  some  shifting  of  the 
divides.  The  sides  of  the  valley  are  steep,  with  many 
cliffs  and  talus  slopes;  small  flood  plains  have  developed 
in  places,  the  river  is  beginning  to  meander;  the  cross 
sections  of  the  valley  are  changing  from  a  V-shape  to  a 
U-shape.  In  the  mature  stage  the  erosion  is  at  the  maxi- 
mum, and  there  is  the  greatest  percentage  of  steep  hill- 
sides over  the  area.  A  large  per  cent  of  the  rainfall  is 
conducted  rapidly  into  the  stream  channels  resulting  in 


90  PHYSICAL  GEOGRAPHY 

destructive  floods  in  the  wet  season.  "  The  tributaries  of 
the  Ohio  River  in  West  Virginia  are  good  examples.  In 
the  mature  stage  the  upland  plains  have  almost  or  entire- 
ly disappeared,  that  is,  they  have  been  dissected  by  the 
stream  and  its  tributaries  into  hills  and  valleys,  the  tops 
of  the  hills  being  the  remnants  of  the  former  upland 
plains  or  plateaus. 


Fig.     65.      Pine     Creek,     Pa.       A     revived     stream    approaching 
maturity  in  the  Alleghany  plateau. 

In  old  age  the  narrow,  sharp  divides  of  the  mature 
stage  are  cut  down  into  low  rounding  hills  with  gentle 
slopes;  the  talus  slopes  extend  to  the  top  of  the  hills,  the 
cliffs  have  disappeared;  flood  plains  increase  in  size  with 
corresponding  increase  in  the  meanders  of  the  river,  and 
formation  of  ox-bow  lakes.  Deltas  increase  in  size  and 
natural  levees  and  river  swamps  become  prominent. 
(Study  the  diagrams  showing  changes  in  profile  and  in 
cross  section,  also  the  contour  maps  cited,  and  the  streams 
seen  in  your  field  trips).     In  extreme  old  age  the  hills  and 


GROUNDWATER  AND  RIVERS  91 

uplands  are  nearly  all  worn  down  to  the  level  of  the  val- 
leys, when  the  whole  area  is  called  a  peneplain,  (Sec.  216) 
the  final  stage  of  erosion  being  that  of  base  level.  (Sec.  70.) 
In  old  age  the  upland  plains  have  disappeared  and  low- 
land plains  have  formed  and  are  increasing  in  size. 
Lakes  and  swamps  are  forming  on  the  flood  plains,  but 
have  all  disappeared  from  the  upland. 


Fig.  66.  Cross  sections  of  a  valley  in 
youth  AA,  maturity  B,  and  old  age  D. 
M  is  a  monadnock.     (See  Section  216.) 

86.  Accidents  or  Interruptions  to  the  Cycle.— The 
cycle  of  erosion  on  a  hard  rock  area  is  so  long  that  there 
is  generally  an  interruption  of  some  sort  before  any  river 
completes  the  whole  cycle.  The  principal  interruptions 
to  the  cycle  are  an  elevation  or  depression  of  the  whole 
or  part  of  the  area  drained  by  the  river.  There  is  abun- 
dant evidence  that  many  land  areas  have  been  elevated  and 
depressed  several  times  during  their  history.  In  a  great 
many  places  at  the  present  time,  the  land  is  slowly  rising 
and  in  other  places  sinking.     (Sec.  217.) 

The  depression  of  the  lower  portion  of  a  river  basin 
carries  part  of  it  below  the  level  of  the  sea  which  extends 
up  the  valley  as  a  bay,  such  as  Delaware  or  Chesapeake 
Bay,  or  an  estuary  such  as  the  Hudson  River  below  Troy. 
The  river  is  dismembered  of  its  tributaries  on  the  drowned 
portion.  Thus  the  Potomac,  Rappahannock,.  York,  and 
James  rivers  that  now  flow  into  Chesapeake  Bay  were 
tributaries  of  the  Susquehanna  River  before  it  was 
drowned.  Study  contour  sheets  for  examples  of  dismem- 
bered rivers. 


92  PHYSICAL  GEOGRAPHY 

If  only  the  middle  or  upper  portions  of  a  river  basin 
are  depressed,  or  depressed  more  than  the  lower  portion, 
the  river  will  permanently  overflow  its  flood  plain,  and 
swamps  or  lakes  will  be  formed. 

87.  Revived  Rivers.— By  the  elevation  of  a  river 
basin  the  stream  and  its  tributaries  are  revived  or  rejuv- 
enated and  enter  upon  a  new  cycle.  The  velocity  of  the 
current  is  quickened,  and  it  begins  to  degrade  and  lower 
its  channel  as  it  did  in  the  beginning.  Where  the  river 
had  reached  old  age  and  was  meandering  on  its  flood 
plain  before  the  elevation  it  will  cut  its  way  down  in  the 
channel  it  occupied  and  intrench  itself  in  the  same  wind- 
ing course  that  it  had  on  the  flood  plain.  See  Canado- 
guinett  Creek  on  the  Harrisburg,  Pa.,  topographic  sheet. 

88.  Superimposed  Rivers.— If  the  elevation  continues 
far  enough,  it  causes  the  river  to  cut  through  the  old  flood 
plain  deposits  and  to  be  superimposed  on  the  hard  rocks 
underneath.  If  the  elevation  proceeds  slowly  or  by 
stages  the  river  will  begin  to  widen  its  new  trench  and 
develop  a  new  flood  plain  at  the  lower  level,  into  which 
the  river  may  again  cut  by  subsequent  elevation. 

89.  Terraces.—  The  remains  of  the  flood  plains  left  along  the 
sides  of  the  valley  of  a  revived  river  form  terraces,  and  the  high- 
est ones  are  the  oldest.     Similar,  but  much  smaller  terraces  may 


Fig.  67,  Alluvial  terraces  formed  by  the  uplift 
of  an  area  and  consequent  downcutting  by 
the  stream.  Terraces  t  t  remnants  of  an 
early  flood  plain,  t'  t'  of  a  later  flood 
plain.     F  is  the  present  plain. 

be  formed  by  the  ordinary  down-cutting  of  the  river  in  its  first 
cycle.  Terraces  of  this  kind  composed  of  sand,  gravel,  and  silt, 
are   called   alluvial   terraces   and   differ   from  the   rock  terraces 


GKOUND WATER  AND  RIVERS 


93 


which  are  caused  by  the  harder  and  more  resistant  rocks  pro- 
jecting as  ledges  or  terraces  along  the  sides  of  the  valley.  (See 
diagrams.) 


Fig.  68.  Rock  terraces,  t  t,  formed  by  harder  layers 
of  rock  projecting  on  the  hillside.  R,  river,  N, 
natural  levee.  2,  3,  4,  former  positions  of  the 
river  channel.  The  stream  is  now  aggrading 
its  valley  and  building  up  its  flood  plain. 

90.  Reversed  Drainage.— The  lower  or  middle  por- 
tion of  a  river  basin  may  be  elevated  more  rapidly  than 
the  upper  portion.  If  it  is  elevated  more  rapidly  than 
the  river  abrades  its' channel,  it  will  form  a  dam  across 
the  valley  and  a  swamp,  marsh,  or  lake  will  be  formed 


Fig.  69.  Rock  terraces  in  the  Alleghany  plateau.  The  coal  and  under- 
lying clay  beds  disintegrate  more  rapidly  than  the  intervening  sand- 
stone beds  which  form  projecting  ledges  or  terraces.  Springs  emerge 
at  the  outcrop  of  the  coal  and  form  hillside  bogs. 

above  the  dam,  similar  to  that  when  the  upper  portion  of 
the  valley  subsides  faster  than  the  lower  portion.  If  the 
elevation  continues  the  water  may  find  an  outlet  over  the 
divide  at  the  headwaters,  cut  a  deep  channel,  and  thus  re- 
verse the  drainage  of  a  large  part  of  the  river  basin.     The 


94  PHYSICAL  GEOGRAPHY 

drainage  is  sometimes  reversed  by  river  piracy  in  the 
shifting  of  divides.  (See  sec.  93.)  Some  of  the  tributaries 
of  the  Ohio  and  Allegheny  Rivers  in  Western  Pennsylvania 
formerly  flowed  northward  into  what  is  now  the  Lake  Erie 
basin.  They  were  reversed  by  the  glacier  which  came  from 
the  north. 

91.  Antecedent  River.— If  the  elevation  of  a  land  area  takes 
place  no  faster  than  the  river  abrades  its  channel,  then  the 
stream  will  saw  its  way  down  through  the  rising  land,  which  may- 
be a  plain,  a  plateau,  a  mountain  range,  or  even  a  mountain  sys- 
tem, raised  thus  across  the  stream  without  diverting  it  from  its 
original  course.  Such  a  stream  is  called  an  antecedent  river  as 
it  was  there  before  the  elevation  occurred.  The  Colorado  River, 
where  it  flows  through  the  Grand  Canyon  is  an  example.  What 
other  examples  can  you  find? 

The  elevation  of  a  coast  indented  with  bays  and  estuaries  will 
produce  engrafted  rivers;  that  is,  the  rivers  that  previously  flowed 
directly  into  the  arm  of  the  sea  will  become  engrafted  on  one 
main  stream  flowing  across  the  newly  uplifted  land. 

The  coming  of  a  continental  glacier,  such  as  once  covered  New 
York  State  and  a  large  part  of  North  America,  would  for  the 
time  being  destroy  all  the  rivers  in  the  area  covered,  and  after 
melting  of  the  ice  many  of  the  rivers  would  begin  to  form  new 
valleys  and  a  somewhat  complicated  system  of  drainage  would 
result.     (Explained  further  in  Chapters   III   and  IV.) 

92.  Water  Gaps  and  Wind  Gaps.— If  a  number  of 
streams  flow  across  the  outcropping  edges  of  hard  and  soft 
layers  as  shown  in  fig.  70,  tributaries  will  develop  on  the 
soft  layers  at  right  angles  to  the  main  stream.  These  are 
called  subsequent  streams.  It  often  happens  that  one  of 
the  main  streams  (as  in  fig.  71)  deepens  its  channel  faster 
than  its  neighbors.  Its  tributaries  will  then  cut  deeper 
and  longer  which  in  turn  aids  in  deepening  the  main 
stream  channel  (how?)  until  in  time  its  tributary  (a) 
cuts  back  until  it  gains  in  succession  the  head  waters  of 
2,  3,  4,  and  5.  Another  tributary  (b)  might  in  a  similar 
way  decapitate  the  captured  branches  of  a  as  shown  in 


GROUNDWATER  AND  RIVERS 


95 


fig.  71.  The  points  where  the  stream  cuts  through  the 
hard  layers  which  now  form  ridges  as  at  W.  W.  are  called 
water  gaps.  The  low  depressions  or  notches  in  the  ridges 
at  PPP  where  streams  formerly  flowed  are  now  called 
wind  gaps  and  are  utilized  for  highways  in  crossing  the 
ridges.  ( Study  the  Harrisburg,  Pa.  or  the  Harper 's  Ferry, 
Va.,  topographic  sheets  for  good  examples  of  water  gaps 
and  wind  gaps.) 


Fig.    70.     An   early    stage    in    river   piracy.      AA    and 
BB   outcropping  edges  of   hard  rocks. 

93.  Migration  of  Divides.— The  shifting  of  the  waters 
from  one  valley  to  another  as  described  in  the  preceding 
paragraph  causes  a  corresponding  shift  in  the  divides  be- 
tween the  valleys.  This  process  of  stream  capture  is 
known  as  river  piracy.  The  shifting  or  migration  of  the 
divides  goes  on  through  youth  and  maturity  until  the 
divides  become  pretty  well  established  in  old  age.  Trace 
out  on  figs.  70  and  71  the  divides  before  and  after  the  shift- 
ing of  the  streams. 


96 


PHYSICAL  GEOGRAPHY 


Examples  of  river  piracy.  Many  examples  of  piracy  and  shift- 
ing divides  may  be  traced  out  on  the  topographic  contour  maps 
with  a  little  care.  In  New  York  good  examples  may  be  found  on 
the   Kaaterskill   and   Plaaterskill    topographic   sheets. 

In  Pennsylvania  the  North  Branch  of  the  Susquehanna  used 
to  flow  into  the  Delaware  River  through  the  Schuylkill  valley  by 
way  of  Wilkes-Barre.  but  a  tributary  of  the  West  Branch  of  the 


Fig.  71.  A  later  stage  of  river  piracy  than  that 
illustrated  in  Fig.  70.  Stream  No.  1  has 
deepened  its  channel  faster  than  2,  3,  4,  and 
5.  Hence  its  tributaries  have  captured  the 
upper  portions  of  the  other  streams  leaving 
wind  gaps  at  P,  P,  P,  P,  and  forming  water 
gaps  at  W,  W.  Stream  1  is  the  pirate,  streams 
2-6  have  been  beheaded. 


Susquehanna  from  Northumberland  cut  back  into  the  softer 
shales  faster  than  the  old  Susquehanna-Schuylkill  could  cut 
down  the  hard  conglomerate  over  which  it  was  flowing,  so  that 
the  Schuylkill  was  decapitated  at  Wilkes-Barre  and  the  upper 
portion  drained  into  the  Susquehanna.  This  change  should  be 
traced  out  on  the  geological  map  of  Pennsylvania  if  one  is  at 
hand. 


GROUNDWATER  AND  RIVERS 


97 


study  fig.  72  and  see  how  the  Shenandoah  River  captured  the 
headwaters  of  Beaverdam  Creek  and  left  a  wind  gap  at  Snicker's 
Gap  in  the  Blue  Ridge. 


// 

TTc 

lii 

7^ 

tX. 

/S 

KL- 

, 

// 

A 

^# 

//or^/3  ^f^j^^^i  r 

M 

-.     S/f/c%efS  Gap 

-^ 

^"^"^^^ 

Fig.  72.  River  Piracy. — B,  present  condition.  A,  probable  condition  ages 
ago.  In  A,  Snicker's  gap  is  a  water  gap  through  which  Beaverdam 
Creek  is  flowing.  In  B  the  gap  is  a  wind  gap  caused  by  the  more  rapid 
downcutting  of  the  Shenandoah  River  enabling  it  to  capture  the  headwaters 
of  the  other  stream.  Shenandoah  River  is  the  pirate,  Beaverdam  Creek 
has  been   beheaded.      (After  "Willis.) 

94.  Streams  in  Arid  Climates.— There  are  some  forms 
of  erosive  action  that  are  characteristic  of  dry  or  arid 
climates.  In  desert  regions  the  little  rain  that  falls 
descends  in  heavy  thunder  showers,  separated  often  by 
long  intervals,  sometimes  several  months,  sometimes  *  sev- 
eral years.  The  long  intervals  of  dry  weather  cause  the 
death  of  all  vegetation  and  the  heavy  rains,  falling  on 
the  bare  soil,  flow  rapidly  into  and  along  the  channel 
ways,  called  wadies  in  the  Sahara  desert,  there  cutting 
deep  trenches  with  steep,  frequently  perpendicular  sides. 

7 


98  PHYSICAL  GEOGRAPHY 

The  wady  is  frequently  used  as  a  highway  by  the  trav- 
eller of  the  desert  because  he  there  finds  some  shade  and 
protection  from  the  hot  scorching  winds  of  the  desert,  and 
further  any  spring  or  water  hole  in  the  region  is  likely 


Fig.  72a.  Arroyo,  near  Kingman,  Arizona.  A  watercourse  in  an  arid  region 
subject  to  great  floods  from  occasional  cloud  bursts,  but  dry  most  of  the 
time.      (D.  T.  McDougal.) 

to  be  there.  It  is  because  these  wadies  are  so  frequented 
by  travellers  that  sometimes  persons  are  drowned  in  them 
by  the  down-rushing  flood  following  one  of  the  sudden 
storjns.  Similar  deep  gullies  cut  by  the  heavy  rainfalls 
in  the  arid  regions  of  the  southwest  United  States  are 
called  arroyos.     (See  fig.  72a.) 

95.  Playas.—In  interior  basins,  that  is,  areas  in  which  the 
rivers  have  no  outlet  to  the  sea,  there  are  in  places  broad  shal- 
low depressions,  probably  formed  by  the  wind,  that  are  covered 
with  water  after  a  heavy   rain,  but   from   which   the   water  is 


GROUNDWATER  AND  RIVERS  99 

evaporated  during  the  long,  dry  seasons.  Such  areas  are  called 
playas,  a  Spanish  word  meaning  shore  or  strand.  A  playa  of  this 
kind  occurs  in  Black  Rock  desert  in  Nevada,  covering  nearly  a 
thousand  square  miles,  which  in  the  wet  season  is  covered  with 
water  a  few  inches  deep  carried  in  by  the  Quinn  River  which  flows 
into  it.  When  the  water  is  agitated,  by  high  winds  it  becomes 
a  lake  of  mud.  During  the  summer  season  the  water  evaporates 
and  the  area  is  covered  with  a  barren  clay  flat.  Deeper  depres- 
sions may  form  salt  lakes.     (See  Chapter  III.) 

REFERENCES 

Groundwater: 

Fuller,  Underground  waters  of  the  U.  S.,  Water  Supply  and 
Irrig.  Papers  No.  114  of  the  U.  S.  Geol.  Survey.  Many 
of  the  other  bulletins  in  the  same  series  contain  ex- 
cellent articles  on  this  subject  and  that  of  rivers. 
Nos.  44  and  67  are  especially  valuable. 

Schlichter,  Crosby,  et  al,  Underground  Resources  of  Long 
Island,  Professional  Paper  No.  44  U.  S.  Geol.  Survey. 

Observations  and  Experiments  on  the  Fluctuations  in  Level 
and  the  Rate  of  Movement  of  Groundwater,  Bull. 
No.  5  Weather  Bureau,  U.  S.  Dept.  of  Agr. 

Hov6y,   Celebrated  American   Caverns,  Robert  Clarke   Co. 
Rivers: 

Russell,  Rivers  of  North  America,  G.  P.  Putnam  Sons,  1898. 

Davis,  Rivers  and  Valleys  of  Pennsylvania,  National  Geog. 
Magazine,  Vol.  I,  1889. 

McGee,  The  Flood  Plains  of  Rivers,  Forum,  April,  1891. 

Gannett,  Profiles  of  Rivers,  U.  S.  Water  Supply  and  Irriga- 
tion Papers,  No.  44. 

Gannett,  The  Flood  of  April,  1897  in  the  Lower  Miss.,  Scot. 
Geog.  Mag.,  Vol.  13,  1897. 

Powell,  Exploration  of  the  Colorado  River,  Washington,  1875. 

Penck,  Valleys  and  Lakes  of  the  Alps,  8th  Int.  Cong.  Geog- 
raphy. 

Tarr,  Watkins  Glen  and  other  Gorges  of  the  Finger  Lake 
Region,  Pop.  Sci.  Monthly,  May,  1906,  Vol.  68,  p.  387. 

MacDougal,  The  Delta  of  the  Rio  Colorado,  Bull,  Am.  Geog. 
Soc,  Jan.,  '06. 

Cole,  Delta  of  the  St.  Clair  River,  Mich.  Geol.  Surv.,  Vol.  IX, 
Pt.  I. 


CHA*PTER  III 
LAKES 

If  one  should  take  the  delightful  water  trip  from 
Duluth,  at  the  head  of  Lake  Superior,  to  Montreal,  he 
would  travel  on  four  of  the  greatest  lakes  in  the  world 
and  on  five  different  rivers,  yet  all  the  way  he  w^ould  fol- 
low the  natural  course  of  the  water  flowing  from  Duluth 
to  the  sea.  He  would  actually  travel  over  a  part  of  one 
great  river,  but  the  lakes  are  so  large  as  to  obscure  the 
importance  of  the  comparatively  small  rivers  flowing  from 
lake  to  lake.  The  connecting  rivers  St.  Mary's,  St. 
Clair,  Detroit  and  Niagara  have  been  given  separate  names 
and  commonly  considered  separate  rivers. 

A  lake  is  a  body  of  comparatively  still  water,  nearly 
surrounded  by  land.  In  some  localities  the  smaller  bodies 
of  water  are  called  ponds.  In  most  of  the  lakes  the  water 
is  fresh,  in  some  it  is  salt,  and  in  others  alkaline. 

96.  Relation  to  Rivers.— Sometimes  a  lake  is  the  head  of  a 
river;  more  frequently  it  is  a  part  of  a  river,  occupying  an  ex- 
panded portion  of  the  valley.  Generally  it  is  so  much  larger, 
wider  and  deeper  than  the  river  that  the  relation  -of  the  two  is 
not  recognized.  Nearly  all  lakes  ha^e  one  or  more  streams  flow- 
ing into  them  and  one  (rarely  two)  flowing  out.  In  most  lakes 
there  is  no  perceptible  current  or  flow  of  water  as  in  the  river. 

While  in  a  moist  climate  a  lake  may  be  the  head  of  a  river, 
in  an  arid  climate  it  may  be  the  terminus  of  a  river.  In  the 
latter  case  the  lake  will  be  salt  or  alkaline. 

97.  Origin  of  Lake  Basins.— Lakes  are  formed  in 
many  different  ways:  (1)  Any  depression  (basin-like) 
which  extends  below  the  water  table  on  a  new  land  area  in 

100 


LAKES 


101 


a  moist  climate  will  soon  fill  with  water  and  form  a  lake. 
Such  depressions  may  be  due  to  inequalities  on  the  sea 
bottom  before  the  uplift,  or  made  during  the  uplift,  or 
may  be  made  subsequently  by  the  action  of  the  wind,  such 
as  the  playas  already  mentioned.     (2)  Lakes  are  formed 


Fig.  73.  Delta  of  the  Trinity  River  in  Galveston  Bay,  Texas.  The  delta  will 
in  time  extend  across  the  bay  and  Turtle  Bay  will  then  be  a  lake  and 
later  a  swamp.  Compare  with  Salton  Sink  in  Fia.  79  where  the  delta 
has  cut  off  the  end  of  the  gulf. 

by  rivers  on  the  flood-plain  (a)  by  cutting  off  meanders, 
the  ox-bow  lakes;  or  (b)  by  building  up  a  natural  levee 
across  the  mouth  of  a  tributary,  as  on  the  lower  Red 
River;  or  (c)  by  a  tributary  building  an  alluvial  fan 
across  the  main  stream.  (3)  A  river  may  build  a  delta 
across  a  gulf  or  bay,  thus  forming  a  lake.  (Figs.  73  and  79. ) 
(4)  Lakes  may  be  formed  by  the  warping  or  twisting 
of  the  earth's  crust  (diastrophic  lakes).     Any  bending  of 


102  PHYSICAL  GEOGRAPHY 

the  crust  that  produces  a  basin-like  depression  will  result 
in  a  lake  in  a  moist  climate.  Lake  Superior  was  formed 
in  part  at  least  in  this  way. 

The  elevation  of  a  portion  of  a  stream  valley  would 
cause  a  lake  in  the  valley  above  the  elevation,  providing 
the  stream  did  not  cut  down  its  channel  as  fast  as  the 
elevation  took  place. 

(5)  Lakes  are  formed  by  glaciers  in  several  ways:  (gla- 


FlG.  74.  View  from  St.  Regis  Mt.  in  the  Adirondack  Mountains,  showing 
numerous  lakes  of  glacial  origin.  There  is  great  irregularity  in  size 
and  shape.  Some  are  due  to  depressions  in  the  glacial  moraine  deposit, 
others  to  depressions  worn  in  the  rock.      (S.  R.  Stoddard.) 

ciers  are  described  in  chapter  IV)  (a).  The  moraine  forms 
dams  across  a  valley,  especially  on  streams  flowing 
towards  the  glacier.  Such  are  the  Finger  Lakes  in  cen- 
tral New  York,  (b)  The  ice  erodes  depressions  in  the 
rock  which  fill  with  water  and  form  lakes  after  the  re- 
treat of  the  ice.  (c)  In  a  heavy  moraine  deposit  there 
will  be  many  kettle-like  depressions  which  form  lakes, 
(d)  Where  the  water  from  the  melting  glacier  flowed 
over  the  edge  of  a  cliff  it  scooped  out  a  basin  at  the 
foot  similar  to  that  at  the  bottom  of  Niagara  Falls.     Sev- 


LAKES 


103 


eral  small  lakes  of  this  kind  occur  in  the  vicinity  of  Syra- 
cuse, New  York.     (See  chapter  IV). 
tzrts"  »»io'  OS'       I  iz2* 


K2 


4/50* 


Fig.  75.  Contour  map  of  Crater  Lake,  Oregon.  The  numbers  on  the  lake 
represent  depth  of  water  in  feet.  Numbers  on  the  contour  lines  in- 
dicate feet  above  sea  level.  Maximum  depth  of  water  1975.  A  caldera 
lake,  due  to  sinking  of  the  center  of  a  volcanic  mountain.  (U.  S.  Geol. 
Survey. ) 

(6)  Volcanoes   may    form    lakes    by    streams    of    lava 
flowing  across  a  valley  and  thus  forming  a  rock  dam;  or 


Fig.    76.     Cross    section    through    Crater    Lake.      Section    through    the    island 
shown  on  FiG.  75.      (U.  S.  Geol,  Survey.) 


104 


PHYSICAL  GEOGRAPHY 


by  the  subsidence  of  the  bottom  of  the  crater  forming  a 
caldera  or  crater  lake.  (See  Crater  Lake  sheet  in  the 
U.  S.  Topographic  Atlas,  No.  2). 

(7)  Earthquakes  sometimes  cause  the  subsidence  of 
considerable  areas  which  fill  with  water  and  form  lakes. 
Several  such  lakes  were  formed  near  the  mouth  of  the 
Ohio  River  in  southeastern  Missouri  in  1811.     Near  the 


FiQ.  77.  Reelsfoot  Lake.  Teim.,  formed  by  subsidence  of  the  area  during 
the  earthquake  of  1811.  Stumps  are  the  remains  of  the  forest  which  was 
submerged   at  that  time.      (M.   L.   Fuller.) 

village  of  Lone  Pine  in  Owen  Valley,  California,  a  lake 
was  formed  by  an  earthquake  shock  in  1872.  Pig.  77  shows 
a  lake  formed  by  the  Mississippi  Valley  earthquake  in  1811. 
(See  Sec.  239.) 

(8)  Landslides  and  avalanches  sometimes  form  dams 
across  valleys,  causing  lakes.  In  1893  a  landslide  esti- 
mated to  contain  about  800,000,000  tons  of  rock  fell  across 


LAKES 


105 


one  of  the  tributaries  of  the  Ganges  River,  and  built  a  dam 
nearly  a  thousand  feet  deep,  which  caused  the  water  to 
back  up  the  Valley  about  four  miles,  and  form  a  lake  of 
that  length. 

(9)  Small  lakes  are  sometimes  produced  by  beavers 
building  dams  across  the  stream.  Many  of  the  *' Beaver 
Meadows''  through  the  northern  United  States  are  the 


i'iu.    id.      iieaver  Dam  seen  from  below.     The  beavers  build  the  obstruction  in 
a  stream  channel  which  produces   a   lake.      (U.    S.   Biological   Survey.) 

remnants  of  beaver  dams  now  partly  or  wholly  filled  by 
vegetation.  (Fig.  78.)  Sometimes  growing  vegetation  be- 
comes sufficiently  dense  to  obstruct  the  stream  channel  and 
produce  a  lake. 

(10)   Small  lakes  are  sometimes  formed  by  the  chemical 
action  of  the  groundwater  dissolving  and  carrying  away 


106 


PHYSICAL  GEOGKAPHY 


large  quantities  of  rock  material,  leaving  depressions  that 
fill  with  water  and  form  lakes.  In  limestone  regions  such 
depressions  frequently  have  an  opening  at  the  bottom  into 
a  cave  and  are  called  sink-holes.  When  the  hole  at  the 
bottom  of  the  depression  becomes  stopped  so  that  the 
water  does  not  get  through,  the  depression  fills  with 
water,  forming  a  small  lake.  In  the  limestone  regions  of 
Kentucky  and  Indiana,  where  surface  water  is  scarce,  the 
farmers  frequently  stop  the  opening  in  the  bottom  of  the 
sinks  with  clay  in  order  to  hold  the  water  for  the  stock. 
(See  fig.  22,  Sec.  49.) 


Fig.  78a.  Natural  rock  dam  on  Pucaswa  River,  Ontario.  View  looking 
up  stream.  The  massive  rocks  in  the  channel  form  part  of  a  dyke  of 
harder  material  than  the  rocks  on  either  side.  Erosion  of  the  softer 
material  has  formed  a  pool  or  lake  above  the  dyke  and  a  shallow 
channel  below. 

(11)  Lakes  or  ponds  of  limited  extent  are  sometimes 
formed  along  stream  courses  where  the  stream  crosses  the 


LAKES 


107 


f»\*'n't 


H    P   tJ,\A 


Fig.  79.  Relief  map  Salton  sink  before  it  was  flooded.  The  gulf  of  California 
at  one  time  extended  northwest  beyond  Indio.  The  distributary  channels 
indicate  the  delta  deposit.  A  large  part  of  the  low  area  north  of  the 
delta  is  now  (1907)  covered  with  water.  The  S.  P.  R.  R.  was  moved 
northeast  to  the  higher  land.  Compare  with  FiG.  73,  which  shows  an 
earlier,  stage  of  a  similar  phenomenon  on  a  smaller  scale. 


108 


PHYSICAL  GEOGRAPHY 


outcropping  edge  of  a  layer  of  hard  rock  that  is  both  over- 
lain and  underlain  by  softer  layers.  The  action  of  the 
weather  and  water  wears  away  the  softer  rock  faster  than 
the  harder  which  then  forms  an  obstruction  or  natural 
dam  across  the  stream.     (Fig.  78a). 

98.  Some  Examples  of  the  Different  Classes  of  Lakes:  — 
(1)  Great  Salt  Lake  and  many  of  the  other  lakes  in  the  Great 
Basin  area  are  remnants  of  lakes  of  the  first  class.  Black  Rock 
desert  on  the  Quin  River  in  Nevada  is  a  good  example  of  the 
playa,  which  is  a  lake  only  in  the  wet  season,  and  a  mud-covered 
plain  the  remainder  of  the  year. 


Fig.  80.  Beach  of  Salton  Sea.  Ancient  beach  at  right.  Present  (1907) 
beach  at  left.  Intermediate  stage  in  the  center.  The  beach  at  the  right 
is  22  feet  above  sea  level.      (D.  T.  McDougal.) 

(2)  Good  examples  of  the  second  class  occur  on  the  flood- 
plains  of  the  lower  Mississippi  River.  Study  the  maps  of  the 
Mississippi  River  Commission.     (See  fig.  57). 

(3)  Salton  Sink  in  Southern  California  was  at  one  time  part 
of  the  Gulf  of  California.  The  Colorado  River  fiowed  into  this 
gulf  at  Yuma  and  depositing  the  great  mass  of  material  eroded 
from  the  canyons,  it  built  out  the  enormous  delta  which  in  time 
extended  entirely  across  the  gulf,  thus  cutting  off  the  northern 


LAKES 


109 


end  and  forming  a  great  salt  lake.  The  river  established  a  chan- 
nel on  the  south  side  of  the  delta,  flowing  into  the  open  gulf,  but 
sometimes  during  high  floods  part  of  the  water  would  overflow 
into  the  sink  on  the  north  side.     (Figs.  79,  80  and  81). 

The  rainfall  was  so  light  on  this  area  and  the  air  so  dry  and 
warm  that  evaporation  took  place  rapidly;  the  lake  dried  up  and 
the  lake  bottom,  formerly  the  gulf  bottom,  became  a  dry  plain  in 
part  covered  with  salt  and  lying,  at  the  lowest  point,  287  feet 
below  the  sea  level. 

The  soil  covering  this  low  plain  is  very  productive  where 
there  is  sufficient  water,  so  a  few  years  ago  an  irrigating  ditch 
was  dug  from  the  river  across  the  delta  plain  and  the  country 
laid  out  in  farms. 

Everything  prospered  at  first,  but  during  a  high  flood  the 
river  became  unmanageable  like  an  untamed  horse  that  has  sud- 


FlG.  81.  New  River  at  Calexico,  February,  11)()7.  Some  of  the  houses 
of  Mexical  visible  on  the  bluff  at  the  left,  the  remainder  of  the  town 
was  swept  away  by  the  river.      (D.  T.  McDougal.) 

denly  discovered  its  power.  The  water  flowing  through  the  ditch 
was  given  a  much  lower  base  than  the  main  river,  hence  it  be- 
came at  once  a  revived  river  and  began  to  cut  down  and  degrade 


110 


PHYSICAL  GEOGRAPHY 


its  channel.  This  cutting  began  during  a  flood  when  the  open- 
ing became  so  large  that  the  engineers  were  unable  to  check  it, 
and  nearly  all  the  water  of  the  river  continued  to  flow  through 
the  irrigating  channel. 

Since  this  inflow  of  waters  threatened  to  flood  all  the  de- 
pressed area,  the  railway  company  whose  road  was  being  des- 
troyed, joined  forces  with  the  irrigation  company  and  at  an 
enormous  expense  completed  a  dam  across  the  ditch  at  the  river. 


Fig.  82.  Lakes  Thun  and  Brienz  were  formerly  one  which  was  severed  at 
Interlaken  by  mass  of  sediment  carried  in  by  the  two  streams  shown  on 
the  sketch.      Interlaken  is  built  on  the   dividing  delta. 

but  in  a  short  time  the  river  cut  a  new  channel  around  the  dam 
and  again  (January,  1907)  poured  its  flood  of  waters  into  the 
sink.  Another  dam  was  completed  and  the  river  again  turned 
into  its  former  channel. 

A  great  cataract,  nearly  a  mile  wide  and  90  to  100  feet  high, 
was  formed  on  the  lower  course  of  the  irrigating  river  and  be- 
fore the  break  was  closed  was  cutting  its  way  back  at  the  rate 
of  one-third  of  a  mile  per  day. 

This  is  one  of  the  most  Sifflcult  and  serious  problems  that 
has  ever  confronted  irrigating  engineers.  If  they  do  not  suc- 
ceed in  keeping  the  river  in  its  old  channel,  what  will  be  the  re- 
sult in  the  Salton  basin?  Suppose  they  flnd  no  means  of  stopping 
the  receding  waterfall,  what  will  be  the  result  when  it  reaches 
the  great  Laguna  dam  above  Yuma?    This  dam  was  constructed 


LAKES 


111 


across  the  Colorado  River  at  a  cost  of  a  million  dollars,  for  pur- 
poses of  irrigation.  With  no  interference  from  man,  what  would 
finally  become  of  the  cataract?  Why  should  this  cataract  recede 
so  much  faster  than  Niagara  Falls? 

The  Alpine  lakes,  Thun  and  Brienz,  in  Switzerland,  were  at 
one  time  united  in  a  single  lake.  Two  streams  flowing  in  from 
opposite  sides  formed  deltas  which  extended  into  the  lake  until 
they  met  in  the  middle  and  thus  cut  the  lake  in  two.  The  town 
of  Interlaken  (meaning,  between  the  lakes)  is  built  on  the 
dividing  delta.     (Fig.  82.) 

99.  The  Great  Laurentian  Lakes.— Upon  the  north- 
ern boundary  of  the  United  States  is  a  chain  of  the  lar- 
gest fresh-water  lakes  in  the  world.  They  formed  a  use- 
ful and  important  highway  to  the  Indian  and  the  pioneer, 
and  are  now  serving  in  the  same  way  for  a  great  and  in- 
creasing inland  commerce.  The  agricultural  products 
and  mineral  wealth  of  the  great  west  find  their  way  in 
large  quantities  over  these  lakes  to  the  eastern  markets, 
while  products  of  the  eastern  coal  fields  and  the  great 
factories  pass  westward  in  return.  The  dimensions  of  the 
Great  Lakes  are  shown  in  the  following  tabulation: 

The  Great  Laurentian  Lakes 


Lake 

Lake 

Lake 

Lake 

Lake 

Ontario 

Erie 

Huron 

Michigan 

Superior 

Area  in  square  miles 

7,240 

9,960 

17,400 

20,200 

31,200 

Length  of  shore  line 

800 

590 

735 

875 

1,300 

Maximum  depth 

738 

210 

730 

870 

1.008 

Average  depth 

300 

70 

210 

335 

478 

Depth  below  sea  level 

491 

149 

289 

406 

Elevation  of  surface 

above  sea  level 

247 

573 

581 

581 

602 

100.  Salt  lakes  are  formed  in  an  interior  basin  which 
has  a  moderate  rainfall,  sufficient  at  least  to  cause  some 
of  the  water  flowing  from  the  surrounding  highlands  to 


112 


PHYSICAL  GEOGRAPHY 


extend  as  far  as  the  lowest  depression  in  the  basin.  The 
area  of  such  a  lake  fluctuates  with  the  seasons  and  the 
climate,  rising  in  the  wet  season  and  sinking  in  the  dry- 
season.     If  the  climate  becomes  more  arid,  the  lake  will 


SQd 


/dOO 


tm 


Fig.   83.      Diagram  showing  comparative  depth  of  the  Great  Lakes. 


decrease  in  size  and  may  even  disappear,  leaving  a  deposit 
of  salt.  If  the  climate  becomes  more  moist,  the  lake  will 
increase  in  size  until  it  fills  the  basin  and  overflows,  when 
the  salt  will  be  carried  out  and  the  lake  become  fresh. 

Great  Salt  LaTce  in  Utah  is  growing  smaller  and  salt  is  being 
deposited  around  the  borders  at  present,  but  at  one  time  it 
covered  a  larger  area  in  the  Great  Interior  Basin  and  overflowed 
to  the  north. 

In  early  geological  times  there  was  a  dry  climate  in  central 
New  York,  probably  as  dry  as  that  in  Utah  to-day,  as  shown  by 
the  great  beds  of  rock  salt  which  were  formed  at  that  time. 

There  are  fresh  water  lakes  in  the  Great  Interior  Basin  of 
the  United  States  and  other  dry  regions,  but  they  have  an  out- 
let. Salt  lakes  are  those  which  have  no  outlet,  that  is,  they  are 
the  last  basin  into  which  the  water  flows  and  from  which  it 
escapes  only  by  evaporation.  The  ocean  is  the  greatest  body  of 
salt  water  and  it  is  the  basin  into  which  most  of  the  great  rivers 
of  the  world  empty. 

Salinas  are  salt  plains,  sometimes  marshes,  sometimes 
dry  plains,  that  were  probably  salt  lakes  but  which  have 
dried  out  from  change  of  climate  or  other  cause.  Some 
of  them  are  marshes  at  one  season  of  the  year  and  dry 
salt  plains  at  other  seasons. 


LAKES  113 

Alkaline  lakes  are  formed  similarly  to  salt  lakes  when 
the  inflowing  streams  carry  more  alkalies  than  salt  in  so- 
lution.    (See  fig.  84.) 


Fig.  84.  An  alkali  lake  on  the  Laramie  plain  18  miles  west  of  Laramie. 
This  lake  has  no  outlet.  The  temporary  streams  flowing  into  the 
lake  after  heavy  rains  carry  some  alkali  which  accumulates  from 
year  to  year  as  the  water  escapes  by  evaporation.      (U.  G.  Cornell) 

101.  Fluctuations  of  Lake  Levels.— The  level  of  some 
lakes  varies  greatly  from  time  to  time  due  to  one  or  more 
of  several  causes,  (1)  One  of  the  most  common  and 
noticeable  changes  in  level  is  due  to  the  change  in  season 
from  wet  to  dry.  During  the  wet  season  the  water  level 
in  the  lake  rises  from  a  few  inches  to  many  feet  in  differ- 
ent lakes.  During  the  dry  season  the  level  sinks  a  cor- 
responding distance  due  to  the  greater  evaporation,  and 
in  arid  districts  the  water  may  even  evaporate  entirely 
from  lakes  that  are  of  considerable  size  in  a  wet  season. 

(2)  Prevailing  winds  sometimes  produce  a  marked 
effect.  With  a  strong  west  wind  continuing  for  several 
days,  the  water  has  been  known  to  rise  as  much  as  fifteen 
feet  at  the  east  end  of  Lake  Erie,  with  a  somewhat  cor- 
responding depression  at  the  west  end. 


114  PHYSICAL  GEOGRAPHY 

(3)  Difference  in  atmospheric  pressure  may  cause  a 
temporary  subsidence  of  the  surface  at  one  place,  and  ele- 
vation at  another.  Thus  a  marked  high  pressure  area  at 
the  west  end  of  Lake  Erie  might  cause  a  sinking  of  the  lake 
surface  there  and  a  rise  of  the  surface  at  the  east  end.  This 
rising  and  sinking  of  the  surface  is  called  a  seiche.  During 
a  seiche  the  water  rises  or  sinks  from  a  few  inches  to  a  few 
feet.     (See  sees  289  and  296.) 

102.  How  Lakes  Disappear.— As  already  stated  lakes 
on  the  upland  are  signs  of  youthful  topography.  They 
are  short-lived  in  comparison  with  rivers,  plains,  moun- 
tains and  other  natural  features.  Lakes  may  disappear 
in  different  ways:  (1)  If  the  stream  that  drains  the  lake 
cuts  its  channel  deep  enough  it  will  drain  the  water  from 
the  lake  basin.  The  bottom  of  Lake  Erie  is  at  about  the 
same  level  as  the  bottom  of  Niagara  Falls;  should  the 
falls  recede  as  far  as  Buffalo,  Lake  Erie  would  be  drained 
and  there  would  be  a  river  flowing  across  what  is  now  the 
lake  bed. 

(2)  The  streams  flowing  into  lakes  carry  sediment 
which  is  deposited  in  deltas  and  distributed  over  the  lake 
bottom  until  the  basin  is  filled  and  the  lake  disappears. 
This  is  one  of  the  most  active  agencies  in  destroying  lakes. 

Deltas  should  be  studied  in  the  vicinity  of  the  school.  Where 
permanent  lakes  are  absent,  deltas  may  be  studied  in  pools  formed 
by  rains.     (See  fig.  82.) 

(3)  Many  lakes  are  slowly  being  destroyed  by  animal 
and  vegetable  matter  which  accumulates  in  sufficient  quan- 
tities to  fill  the  basin.  Small  molluscs,  commonly  known 
as  periwinkles,  grow  in  great  numbers  in  some  of  the  small 
lakes  and  as  they  die  their  shells  accumulate  on  the  lake 
bed  forming  bodies  of  shell  marl  which  occur  in  some 
places  fifty  feet  or  more  in  depth.  Marl  is  frequently 
composed    partly    of    plant    remains.      Vegetation    grows 


LAKES 


115 


on  the  bottom  of  lakes  and  around  the  margin.  In  the 
small  lakes  in  cold  climates  a  plant  known  as  the  sphag- 
num or  pea^  plant  grows  in  great  luxuriance  even  on  top 
of  the  water;  the  remains  of  this  vegetation  accumulates 
on  the  lake  bed  as  peat,  or  muck  until  it  finally  fills  the 
lake  basin.    Frequently  the  filling  of  the  basin  is  due  to 


FlQ,  85.  Upper  Avisablc  lake  in  the  Adiroiuhiek  Aiouutains.  The  laKe 
is  being  filled  by  vegetable  matter.  The  shrubby  growth  at  the  sides 
is  on  the  part  completely  filled.  The  shrub  area  is  extending  towards 
the  center  of  the  lake.  The  bordering  forests  are  crowding  upon  the 
shrub  area.  Notice  the  zonal  arrangement  of  the  vegetable  growth. 
Part  of  the  bordering  flat  is  a  quaking  bog.      (S.  R.  Stoddard.) 


the  combined  action  of  these  two  agencies.  Many  of  the 
peat  and  muck  areas  (vegetable)  are  underlain  by  marl 
(animal)  deposits.  Hundreds  of  small  lakes  in  New- 
York  and  elsewhere  have  been  destroyed  in  this  way.  (Fig. 
85.) 

(4)  Lakes  may  be  destroyed  by  volcanic  action  in  one 


116 


PHYSICAL  GEOGRAPHY 


of  two  ways:  (a)  by  material  ejected  from  a  volcano  fill- 
ing the  basin;  or  (b)  as  in  the  case  of  the  lake  on  Mount 
Pelee,  Martinique,  the  eruption  takes  place  underneath 
the  lake  and  blows  it  out  of  existence. 

(5)   Winds  also  assist  in  filling  lake  basins  by  blowing 
in  sand  and  dust  from  the  surrounding  area. 


H 

, 

^^,. 

^^S 

^S 

^^^^^3 

t|||!li||| 

^^i^l^^P 

1  '^  '^ 

C 

1           ■■-  : 

Gilbert's  map  of  glacial  lake  Iroquois 

Fig.    86.      The    dotted    line    on    the    shaded    area    shows    the    present   boundary 

of  Lake  Ontario.     The  greater  lake  Iroquois  drained  through  the  Mohawk 

valley   because    the   glacier    then    filled    the    St.    Lawrence    valley    and    pre^ 

vented  the  water  from  flowing  through  that  valley  as  it  does  at  present. 

103.  Fossil  Lakes.— Areas  formerly  covered  by  lakes  that 
have  been  filled  or  drained  by  some  means,  are  classed  as  fos- 
sil lakes,  and  they  may  be  recognized  by  the  following  marks: 
(1)  By  characteristic  shore  features  or  markings,  such  as  cobble 
or  gravel  beaches,  or  sand,  gravel  or  wave-cut  terraces.  (Des- 
cribed in  Chapter  VI  on  Shore  Features)  (2)  By  characteristic 
lake-bed  deposits  such  as  peat,  marl,  diatomaceous  earth,  (see 
sec.  105)  and  bog  iron  ore.  Lake  Iroquois  at  the  close  of  the 
glacial  period,  covered  a  large  area  south  and  east  of  Lake 
Ontario,  an  area  now  bordered  by  shore  features  and  covered 
with  lake-bed  deposits.*     (Fig.  86.) 

*  Fossil  Lake  Passaic  in  New  Jersey  is  described  in  the  Annual  Report 
New  Jersey  Geological  Survey,  1893,  and  fossil  Lake  Agassiz,  northwest  of 
Lake  Superior,  is  described  in  a  large  monograph  of  the  U.  S.  Geological 
Survey. 


LAKES 


117 


Fig.  87.  View  in  a  gravel  quarry  on  Fossil.  Lake,  Iroquois  beach  near  Wolf 
Street  in  Syracuse.  Notice  the  characteristic  beach  structure  in  the 
gravel,  the  slope  of  the  layers  from  right  to  left.  The  island  shore  line 
was  on  the  right. 

104.  Life  in  Lakes  and  Rivers.— All  the  rivers  and 
most  of  the  lakes  except  the  salt  and  alkaline  lakes  contain 
many  forms  of  animal  and  vegetable  life.  The  animal 
life  is  probably  more  prolific  in  the  larger  lakes  and  the 
vegetable  life  more  abundant  in  the  smaller  and  shallower 
lakes.  As  already  stated  many  of  the  smaller  lakes  ter- 
minate by  being  filled  with  the  accumulated  remains  of 
the  animals  and  plants. 

Some  animals,  such  as  eels  and  salmon  spend  part  of 
their  existence  in  salt  water  and  part  in  fresh ;  but  with  a 
few  exceptions  of  this  kind,  the  life,  both  animal  and  plant, 
in  the  rivers  and  lakes  is  decidedly  different  from  that  in 
the  sea.  The  life  of  the  sea  is  more  varied  and  locally  more 
prolific  than  in  the  lakes. 

105.  Diatoms.— There    is    one    class    of    exceedingly    small., 


118 


PHYSICAL  GEOGRAPHY 


microscopic  plants  known  as  diatoms,  that  are  found  abundant- 
ly in  both  the  lakes  an^  the  ocean.  The  diatoms  are  composed 
of  opal,  that  is,  silica,  combined  with  water.  So  small  are  these 
plants  that  a  German  scientist,  Ehrenberg,  has  estimated  that 
there  are  about  four  billions  of  them  in  a  cubic   inch.     Yet  so 


Fig.    88.     Micro-drawing    of    diatoms    from    the    mountains    at    Lompoc,    Cal. 
(W.   F.   Prouty.) 

numerous  are  they  that  in  some  places  they  form  deposits  many 
hundreds,  even  thousands  of  feet  in  thickness.  The  material 
composed  of  diatoms  is  known  as  tripoli  or  diatomaceoufs  earth. 
It  is  so  light  and  porous  that  it  floats  on  water.  It  is  used  as  a 
polishing  powder,  as  a  filler  for  soaps,  as  an  absorbent  for  nitro- 


LAKES 


119 


glycerine  in  making  dynamite,  and  as  fire-proof  material  in 
buildings.  A  slippery  brown  material  that  covers  the  stones  in 
the  brook  in  many  places  is  composed  of  diatoms.  The  large 
deposits  of  diatoms  in  California  are  in  some  places  snow  white, 
in  others  colored  by  the  impurities  mingled  with  them.  It  is 
estimated  that  an  area  of  more  than  10,000,000  square  miles  of 
the  sea  bottom  is  covered  with  diatomaceous  deposits.  (See 
figs  88  and  89.) 


Fig.  89.  View  of  mountains  formed  by  a  white  diatom  deposit  at  Lompoc,  OaL 
The  deposit  composed  entirely  of  diatom  remains  is  more  than  1000  feet 
thick,   and  covers  an   area  of  many  square  miles. 


106.  Functions  of  Lakes.— Lakes  have  a  number  of 
important  functions  which  are  directly  or  indirectly  of 
commercial  importance  to  man  in  his  varied  industries. 
(1)  They  serve  as  a  regulator  of  floods.  A  river  like  the 
St.  Lawrence  with  many  large  lakes  in  its  course  is  never 
troubled  with  such  destructive  floods  as  visit  the   Ohio 


120  PHYSICAL  GEOGRAPHY 

River  which  has  almost  no  lakes  in  its  basin.  The  rain 
that  falls  in  the  upper  St.  Lawrence  basin  flows  into  the 
Great  Lakes  where  it  spreads  out  over  thousands  of 
square  miles  of  lake  surface  with  almost  no  effect  on  the 
Niagara  or  St.  Lawrence  rivers  below,  while  the  rain  that 
falls  in  the  upper  Ohio  Valley,  having  no  lakes  in  which 
to  spread,  runs  rapidly  into  the  river  channels  causing 
great  floods.  In  flood  season  the  Ohio  River  has  been 
known  to  rise  50  to  60  feet  above  low  water  while  a  rise 
of  half  as  many  inches  is  rare  in  the  St.  Lawrence.  (2) 
They  form  a  valuable  water  supply  for  cities.  (3)  They 
serve  as  highways  of  navigation.  The  commerce  on  the 
lakes  on  our  northern  boundary  now  reaches  quite  ex- 
tensive proportions.  (4)  They  form  valuable  fishing 
grounds.  (5)  The  larger  lakes  form  excellent  sites  for 
cities  and  the  smaller  ones  for  summer  resorts.  (6)  They 
temper  the  climate  in  their  vicinity.  (7)  They  furnish 
a  constant  and  steady  supply  of  water  to  the  rivers  flow- 
ing from  them.  They  serve  as  settling  tanks  for  the 
rivers.  The  waters  flow  in  muddy,  the  mud  settles  and 
the  stream  flows  out  clear. 

107.  Life  History  of  Lakes.— Like  other  natural 
phenomena,  a  lake  has  a  period  of  growth,  maturity,  de- 
cline, old  age,  and  death,  which  may  be  termed  its  life- 
history.  This  is  not  uniform  but  varies  with  different 
classes  of  lakes.  The  greatest  variation  is  between  lakes 
in  dry  and  those  in  moist  climates.  In  moist  climates  the 
average  life  of  the  lake  is  not  long  in  comparison  with  the 
life  of  a  river  or  mountain,  but  much  longer  than  the  life 
of  any  animal  or  plant. 

Lakes  may  come  into  existence  in  any  one  of  the  sev- 
eral ways  mentioned  in  the  preceding  pages.  With  the 
exception  of  a  very  few  lakes  that  have  been  blown  up  by 
volcanic  explosions,  they  are  destroyed  slowly  by  the  com- 


LAKES  121 

bined  action  of  the  different  agencies  previously  described. 
When  a  lake  basin  is  filled,  the  lake  disappears  as  a  body 
of  water  and  the  streams  meander  across  the  fertile  plain 
formed  of  lake  deposits.  In  the  course  of  time  the  stream 
leading  away  from  the  old  lake  basin  will  lower  its  chan- 
nel, because  it  now  carries  sediment  which  was  formerly 
deposited  in  the  lake.  This  will  cause  the  streams  flowing 
over  the  plain  of  the  lake-filled  basin,  to  quicken  their 
velocity  and  hence  erode  the  soft  materials,  finally  carry- 
ing away  all  the  deposits  that  filled  the  lake,  thus  destroy- 
ing the  last  vestiges.  The  streams  on  the  Fargo,  North 
Dakota,  topographic  sheet  are  flowing  over  a  lake-filled 
plain  and  are-  just  beginning  the  work  of  carrying  away 
the  sediment.     (See  figs.  47  and  48.) 

The  lake  is  thus  seen  to  be  an  incident  in  the  life  of  a 
river  which  deposits  in  the  lake  bed  the  load  of  sediment 
it  is  carrying  from  the  mountains  to  the  sea.  At  a  later 
period  after  having  filled  the  lake  basin,  it  again  takes  up 
the  sediment  and  carries  it  on  to  the  next  stopping  place 
and  finally  to  the  sea,  the  largest  lake  of  all. 

Many  small  lakes  have  a  different  history  from  the  above, 
because  they  have  scarcely  any  sediment  carried  into  them;  in 
fact,  there  are  many  small  lakes  that  have  no  surface  streams 
flowing  into  them  and  either  no  stream  or  else  a  very  small, 
sluggish  stream  flowing  out.  (See  fig.  90).  Such  lakes  will  be 
filled  in  tim^  by  the  remains  of  plants  and  animals  that  grow  and 
die  in  the  lake.  Such  a  lake  forms  first  a  swamp  which  later 
becomes  solid  and  forms  a  meadow  or  vly  as  it  is  called  in  the 
region  of  the  Adirondacks.  There  are  no  deltas,  beaches,  or 
evidences  of  wave  action  in  most  of  these  small  muck-and-marl- 
filled  lakes. 

108.  Lakes  in  Arid  Regions.— In  arid  regions  the  life 
history  of  the  lake  is  somewhat  different,  being  in  general 
longer  and  more  complex  than  in  a  humid  region.  In  an 
enclosed  basin-area   the   lowest   depressions  will  have   no 


122  PHYSICAL  GEOGRAPHY 

outlet  and  hence  cannot  be  destroyed  by  draining.  The 
sediments  and  salts  carried  in  and  deposited,  raise  the 
lake-bed,  but  this  in  turn  raises  the  level  of  the  water  and 


Fig.  90.  Glacial  lake  near  Pilot  Harbor,  Ontario.  The  end  of 
the  lake  at  the  left  has  been  filled  and  is  now  covered  with 
swamp  grass  and  shrubs.  On  the  middle  portion  the  forest 
grows  to  the  water's  edge.  The  lake  will  in  time  be  filled 
with  vegetable  matter  and  form  a  meadow. 

causes  it  to  spread  out  over  a  greater  area.  But  increased 
area  means  increased  evaporation  and  hence  the  lake  in- 
creases or  diminishes  as  the  case  may  be,  until  the  evapo- 
ration equals  the  inflow.  The  lake  will  be  subject  to  many 
vicissitudes  with  change  of  climate,  until  it  finally  ends 
in  one  of  two  ways:  (a)  aridity  may  increase  until  the 
lake  dries  up  and  disappears  as  such  until  there  is  another 
change  in  climate;  or  (b)  the  humidity  may  increase  un- 
til the  entire  basin  fills  up  and  overflows  when  it  ends  as 
any  other  lake  in  the  humid  region.  Lakes  may  become 
salt  in  a  humid  region  by  being  depressed  below  sea  level 
where  the  sea  water  has  access  to  the  basin,  when  it  be- 
comes for  a  time  an  arm  of  the  sea. 


LAKES  123 

Following  the  glacial  period,  Lake  Champlain  was  an  arm 
of  the  sea  which  extended  up  the  St.  Lawrence  Valley  and  filled 
the  Champlain  Valley  130  feet  above  the  present  level  of  the 
lake.  A  later  elevation  of  the  land  drained  the  lake  to  its 
present  level  and  the  fresh  water  streams  flowing  into  and 
through  it  carried  the  salt  to  the  sea,  thus  changing  it  to  a  fresh 
water  lake. 

109.  Swamps  and  Marshes.— Swamps  are  for  the 
most  part  closely  associated  with  lakes  and  rivers  and 
are  likewise  a  kind  of  connecting  link  between  the  water 
and  land  areas.  There  are  both  fresh  water  and  marine 
marshes.*  The  fresh  water  swamps  may  be  conveniently 
divided  into  river,  lake,  and  upland  swamps. 

The  river  swamps  may  be  divided  into  terrace  and 
flood-plain  swamps.  The  terrace  swamps,  sometimes 
called  hillside  bogs,  are  formed  by  the  outcrop  on  the  hill- 
side of  a  bed  of  clay,  shale,  or  similar  rock  that  causes  a 
continual  seepage  of  the  groundwater  into  the  clay  soil  on 
the  surface.  They  are  abundant  in  the  bituminous  coal 
fields  of  the  Appalachian  plateau  where  they  are  caused 
by  the  outcrop  of  the  clay  beds  that  underlie  the  coal 
seams.  The  flood  plain  swamps  are  in  great  numbers  on 
nearly  every  river  flood  plain  and  delta  as  already  de- 
scribed.    (See  figs.  58  and  69.) 

Lacustrine  or  lake  swamps  are  of  two  classes:  those 
formed  on  the  lake  margin  caused  by  a  rise  and  overflow 
of  the  lake  or  by  the  elevation  to  the  surface  of  all  or  part 
of  the  lake  bottom  through  the  accumulation  of  vegetable 
or  animal  remains,  such  as  the  shell  deposits  which  form 
the  marl.  The  second  class,  known  as  quaking  hogs,  is 
caused  in  the  final  stages  of  lake-filling  by  vegetation, 
when  the  floating  plants  on  the  surface  join  those  growing 
out  from  the  shore,  forming  a  continuous  surface  of  vege- 

*  There  is  a  tendency  at  present  to  use  the  word  "swamp"'  for  the  fresh 
water  forms   and   "marshes"   for  the  marine. 


124 


PHYSICAL  GEOGRAPHY 


tation  across  the  remnant  of  lake  water  underneath.  The 
climbing  hog  is  formed  by  the  vegetation  drawing  the 
water  by  capillary  attraction  above  the  level  of  the  lake, 


Fig.  91.  Climbing  bog,  L,  lake.  B,  bog.  PP,  peat  plant  growing  on  float- 
ing vegetable  remains.  M,  muck  accumulating  on  the  bottom.  C, 
climbing  bog.      (After   Shaler.) 

extending  the  bog   above   and   beyond    the    former   lake- 
shore.    (See  figs.  91  and  92.) 

Upland  swamps  may  be  formed  on  clay  soils  by  the 
accumulation  of  plant  remains  which  prevent  the  rapid 
drying  out  of  the  moisture  while  the  underlying  clay  re- 
tards its  descent  as  groundwater.  Such  swamps  some- 
times build  up  many  feet  above  the  level  of  the  surround- 
ing flat  on  which  they   are  located,  and  at  times  after 


Fig.  92.  Bog  and  swamp  formed  by  lake  filling.  DD,  diatomaceous  de- 
posit. II,  iron  ore.  PP,  peat.  S,  swamp.  B,  quaking  bog.  C, 
climbing  bog.     W,  remnant  of  lake  not  yet  filled.      (After  Shaler.) 

heavy  rains,  have  been  known  to  burst  and  flood  the  ad- 
joining areas  with  a  mass  of  black  mud  or  muck  formed 
by  the  decaying  vegetation. 

110.  Salt  Marshes.— Besides  the  fresh  water  swamps 
there  are  vast  areas  of  salt  marshes  along  the  seaboard. 
(See  chapter  on  Shore  Lines).  Professor  Shaler  esti- 
mated that  there  are  at  least  350,000  acres  of  marsh  land 


LAKES 


125 


between  New  York  City  and  Portland  and  that  200,000 
acres  of  this  could  be  reclaimed,  drained,  and  made  into 
agricultural  land  that  would  have  a  value  of  $40,000,000. 

111.  Economic  Features  of  Swamps  and  Marshes.— Swamps 
and  marshes  are  not  entirely  barren  stretches.  Among  the 
many   economic   products   from  them   might   be  named  the  fol- 


FlG.  93.  Marl  deposit  in  former  lake  bed  at  Wariiprs,  N.  Y. 
now  being  quarried  for  use  in  the  manufacture  of  Portland 
cement.      (S.    H.    Ludlow.) 

lowing:  rich  agricultural  land  after  drainage,  timber,  peat,  cran- 
berries, tripoli,  marl,  phosphates,  bog  iron  ore,  regulation  of 
streams,  game.  What  other  economic  products  or  features  of 
swamps  can  you  name?  Enumerate  some  of  the  undesirable 
features  of  swamps. 


FALLS  AND  RAPIDS 


112.  A  stream  flowing  from  a  lake  at  one  level  to 
another  at  a  lower  level,  or  in  flowing  from  an  upland  to 
a  lowland  valley  will  have  rapids  or  falls  at  the  abrupt 
descents  in  the  course.     Lake  Superior  is  21  feet  higher 


126  PHYSICAL  GEOGRAPHY 

than  Lake  Huron  and  the  St.  Mary's  River  which  con- 
nects the  two  descends  this  distance  over  the  rapids  at 
the  very  outlet  of  Lake  Superior.  Lake  Erie  is  326  feet 
above  Lake  Ontario  and  the  connecting  river  descends 
half  of  this  distance  suddenly  at  the  falls  in  the  Niagara 
River  and  the  other  half  mostly  in  the  rapids  above  and 
below  the  falls. 

Falls  or  rapids  may  come  into  existence  in  a  stream 
where  its  course  leads  it  over  a  cliff  or  steep  slope,  or  they 
may  develop  on  a  stream  course  which  was  at  first  uni- 
form, providing  there  are  one  or  more  hard  layers  of  rock 
separated  by  softer  layers  all  lying  horizontal  or  inclined 
up  stream.  Where  the  stream  flows  across  the  outcrop- 
ping layers  of  the  ledges,  the  underlying  softer  one  will 
be  eroded  more  rapidly  than  the  overlying  harder  one 
which  in  time  is  undermined  until  the  overhanging  por- 
tion breaks  down  of  its  own  weight  and  thus  causes  the 
falls  to  move  slowly  up  stream.  This  process  will  con- 
tinue until  the  grade  of  the  stream  brings  the  bottom  of 
the  channel  above  the  top  of  the  soft  layer,  when  the  falls 
will  change  to  rapids,  and  the  rapids  recede  until  they 
are  graded  and  thus  disappear. 

113.  Niagara  Falls.— The  Niagara  River  flows  over 
the  Lake  Erie  plain  from  the  lake  to  the  falls  where  it 
drops  160  feet  into  the  gorge  through  which  it  flows  un- 
til it  emerges  on  the  Lake  Ontario  plain  at  Lewiston, 
seven  miles  below.  Above  the  falls  the  river  flows  over 
the  Niagara  limestone  which  also  forms  the  cap  rock  on 
both  sides  of  the  gorge.  Underlying  the  hard  limestone 
is  a  bed  of  softer  shales  which  is  more  readily  eroded  by 
the  water  than  the  hard  limestone  at  the  top.  The  water 
plunging  into  the  pool  at  the  base  of  the  falls  wears  away 
the  softer  shales  and  leaves  the  limestone  projecting  as 
an  overhanging  ledge  until  it  breaks  off  by  its  own  weight. 


LAKES 


127 


causing  the  falls  to  recede  the  width  of  the  fallen  mass. 
The  repetition  of  the  process  has  caused  the  falls  to  move 
back,  thus  lengthening  the  gorge  more  than  three  hundred 
feet  since  the  first  measurement  was  taken  in  1842.  (Study 
figures  94  and  95,  and  the  U.  S.  topographic  map  of  Niagara 
Falls.) 


Fig.  94.     Part  of  Niagara  Falls  and  gorge.    View  from  Goat  Island.    Notice 
the  shallowness  of  the  water  near  the  American  shore.      (E.  R.  Smith.) 

The  continuation  of  this  process  in  the  past  has 
caused  the  river  to  cut  the  gorge  all  the  way,  about  seven 
miles,  from  Lewiston  to  the  present  position,  and  if  the 
movement  continues,  the  falls  will  eventually  be  carried 
back  to  Lake  Erie  and  the  lake  will  be  drained.  How- 
ever, it  is  probable  that  the  falls  will  change  to  rapids 
and  new  falls  develop  on  higher  beds  before  Lake  Erie 
is  drained. 

If  the  rate  was  uniform  in  the  past,  as  at  present,   (about 


128 


PHYSICAL  GEOGRAPHY 


five  feet  per  year),  how  old  are  the  falls  now?  How  long  will 
it  be  until  they  reach  Lake  Erie  at  the  same  rate?  (See  the 
contour  map  sheets  of  this  region  for  distances.)  Can  you  infer 
from  the  diagrams  showing  the   position  of  the  rocks,  whether 


Pia.  95.  Vertical  section  at  Niagara  Falls.  The  softer  shales  underneath 
are  worn  away  by  the  water.  The  overlying  hard  limestone  breaks  oflf 
and  is  carried  down  the  gorge.  The  repetition  of  this  process  causes 
a  recession  of  the  falls  and  the  lengthening  of  the  gorge.  (After 
Gilbert. ) 

the  rate  of  recession  has  been  increasing  or  decreasing,  and 
how  it  will  be  in  the  future?  What  difference  would  it  make 
if  the  Niagara  limestone  were  near  the  surface  of  Lake  Erie  in- 
stead of  below  the  bed  of  the  lake? 


LAKES 


129 


The  rapids  on  the  St.  Mary's  River  will  recede  in  time  until 
the  surface  of  Lake  Superior  is  lowered  to,  or  near  the  level  of 
Lake  Huron,  but  the  recession  will  be  very  slow,  because  the 
water  flowing  out  of  the  lake  carries  little  or  no  sediment  and 
hence  no  graving  tools  to  cut  away  the  rock  over  which  it  is 
flowing.  The  rock  being  a  hard  sandstone  is  not  soluble,  so  that 
it  cannot  be  carried  away  in  solution. 

114.  Falls  Formed  by  Frost.— At  the  Stone  Quarry 
Falls,  near  Manlius,  New  York,  there  is  a  hard  limestone 


Fig.  96.  Stone  Quarry  Falls  at  Manlius,  N.  Y.  The  falls  pro- 
ject into  the  gorge  with  a  recession  on  each  side.  The  dis- 
integration by  the  frost  on  each  side  of  the  falls  is  more 
rapid  than  the  wear  of  the  rocks  by  the   water. 

rock  at  the  top  of  the  falls  underlain  by  shaly  limestone 
and  shale  similar  to  Niagara  Falls,  except  that  the  lime- 
stone is  thicker  in  proportion  to  the  shale.  Here  the  water 
does  not  undermine  the  limestone  by  wearing  away  the 
softer  shale  but  the  shale  projects  out  beyond  the  lime- 
stone and  forms  a  rounded  prominence  at  the  head  of  the 
gorge  over  which  the  water  descends  by  successive  stages 
9 


130 


PHYSICAL  GEOGRAPHY 


from  layer  to  layer.  The  middle  and  foot  of  the  falls 
project  several  feet  beyond  the  top  instead  of  the  reverse 
as  at  Niagara  Falls.     (See  fig.  96.) 

At  the  Stone   Quarry  Falls,  the  gorge  is  wider  than 
the  falls  and  there  is  a  recession  at  the  head  of  the  gorge 


Fig.  97.  View  in  Havana  Glen,  N.  Y.,  illustrating  the  effect  of  joint  planes 
on  the  direction  of  the  stream  and  bordering  cliffs.  Near  the  middle  of 
the  picture  the  stream  makes  a  turn  at  right  angles  in  changing  from  one 
system  of  joint   planes  to  another. 

on  each  side  of  the  projection  under  the  falls.  The  ex- 
planation is  found  in  the  relatively  greater  work  of  the 
frost.  Under  the  falls  the  rock  is  protected  by  the  run- 
ning water  which  does  not  freeze;  at  the  sides,  which  are 
moistened  by  the  spray,  the  frost  splits  off  fragments, 
causing  the  greater  recession.  Instead  of  erosion  under 
the  falls  there  is  even  deposition  at  times  of  some  carbon- 


LAKES 


131 


132 


PHYSICAL  GEOGRAPHY 


ate  of  lime  from  that  held  in  solution  by  the  water  in  the 
stream. 

In  some  places  waterfalls  recede  by  the  splitting  off 
of  blocks  along  the  vertical  joint  planes.  Joint  planes  are 
natural  planes  of  parting,  generally  vertical,  that  intersect 
all  rocks,  but  are  especially  prominent  in  some  sedimentary 
formations.  Where  the  bottom  of  the  cliff  is  eroded,  the 
overhanging  portions  break  away  along  these  planes  and 
fall  in  huge  blocks  leaving  the  smooth  vertical  walls  of  the 
joint  plane.  These  sometimes  influence  the  direction  of  a 
stream  at  the  falls.     (See  fig.  97) 


Fig.  99.  Montour  Falls  near  Watkins  Glen,  N.  Y.  The  water  flows  over  a 
prominence  with  a  depression  on  each  side  formed  mainly  by  action  of  the 
frost.     What  causes  the  vertical  face   at  the  base  of  the  falls? 


At  Barnett  Falls,  Vermont,  as  shown  in  fig.  98  the 
stream  is  being  turned  from  its  present  course  by  the 
natural  cleavage  planes  in  the  rocks.     There  are  many  ex- 


LAKES 


133 


amples  of  falls  of  this  kind  in  AVatkins  and  Havana  Glens, 
and  elsewhere  in  the  Finger  Lake  region  of  New  York. 

Other  natural  planes  of  parting  in  the  rock  sometimes 
have  an  effect  similar  to  that  of  joint  planes.  (See  fig.  98.) 

How  many  of  the  waterfalls  that  you  have  seen  belong 
to  the  Niagara  type?  Can  you  explain  the  origin  of  any 
of  the  others  that  are  of  a  different  type?  Montour  Falls 
near  Watkins  Glen  belong  to  the  Stone  Quarry  type. 
(See  fig.  99.) 


Fia.  100.  Tinker  Falls  near  Tully,  N,  Y.  The  water  falls  from 
the  hard  Tully  limestone  upon  the  softer  Hamilton  shales.  The 
deep  recession  back  of  the  falls,  30  feet  or  more,  is  due  to  the 
action  of  frost  and  other  weathering  agencies. 

Falls  and  rapids  may  be  formed  in  other  ways  than 
those  described  above,  but  the  ones  mentioned  are  typical 
of  hundreds  of  similar  examples  scattered  over  New  York 
and  other  parts  of  the  United  States. 

Waterfalls  are  more  numerous  in  the  Northern  United 
States  than  in  the  central  and  southern  portions,  because  of  the 
action  of  the  glacier  that  formerly  covered  the  area.  The  gla- 
cier by  deepening  and  widening  many  of  the  larger  valleys  and 


134 


PHYSICAL  GEOGRAPHY 


removing  the  talus  material  at  the  bottom  of  the  hill  slopes, 
caused  the  smaller  tributary  streams  to  enter  the  main  valleys 
over  cliffs,  thus  producing  cataracts.  These  are  characterized 
as  hanging  valleys  (see  sec.  136).  In  some  instances  the  trib- 
utary valleys  were  filled  by  glacial  debris  causing  the  stream  to 
form  a  new  channel.  The  new  course  of  the  stream  frequently 
led  it  over  a  cliff,  resulting  in  a  waterfall. 


/    Li]-me»itQ'nel 


Fia.  101.  Vertical  section  at  Tinkers  Falls,  N.  Y.  The  soft  shale  bed  near 
the  middle  of  the  section  disintegrates  more  rapidly  than  the  other 
rocks  and  thus  makes  the  deep  notch  back  underneath  the  falls.  During 
the  dry  season  the  stream  of  clear  water  falls  at  L  and  does  little 
eroding.  During  high  water,  the  larger  stream  carrying  sediment 
falls  at  H  wearing  away  the  lower  end  of  the  talus,  even  wearing  a 
basin  in  the  bed  rock  at  H. 


Reaches.  On  many  rivers  like  the  Genesee  in  New  York 
and  the  Yellowstone  in  the  National  Park  there  are  several 
waterfalls  separated  by  more  or  less  graded  reaches  similar 


LAKES  135 

to  those  which  separate  the  rapids  at  a  later  period.  (See 
sec.  69).  These  reaches  may  vary  in  length  from  a  few 
inches  to  several  miles. 

115.  Economic  Importance  of  Waterfalls.— The  solar 
energy  that  lifts  the  water  from  the  sea  into  the  atmos- 
phere is  stored  in  the  raindrops  and  this  is  concentrated 
at  the  waterfalls  in  such  a  way  that  man  can  utilize  it  to 
turn  the  wheels  of  his  machinery  and  thus  turn  it  into 
mechanical  energy,  heat,  or  light. 

In  the  early  settlements  of  North  America  the  vicinity 
of  the  waterfalls  was  the  point  first  selected  by  the 
pioneer  for  his  home  and  especially  for  his  villages  and 
towns,  because  here  he  found  the  energy  to  run  the  mills, 
to  grind  his  corn  and  saw  his  lumber.  Later  when  steam 
power  was  discovered  and  utilized  and  the  great  beds  of 
coal  were  found  in  Pennsylvania  and  elsewhere,  steam 
was  used  to  run  the  factories  and  the  waterwheels  were 
neglected  in  many  places. 

Recent  improvements  in  electrical  appliances,  by  which  the 
energy  in  the  waterfall  can  be  cheaply  transported  on  metallic 
wires  long  distances  and  turned  into  mechanical  power,  heat, 
or  light  as  desired,  emphasize  again  the  importance  of  water- 
falls, poetically  called  "White  Coal,"  and  the  power  is  now  being 
used  in  hundreds  of  places.  Energy  from  Niagara  Falls  is 
carried  by  wire  into  central  New  York  to  light  the  cities,  run 
the  cars,  and  furnish  power  for  some  of  the  factories.  What 
other  falls  can  you  name  that  are  utilized  in  a  similar  way? 

116.  The  Fall  Line.— Along  the  Atlantic  sea  coast 
is  a  coastal  plain  of  varying  width  which  is  underlain  by 
beds  of  sand,  clay,  and  gravel  that  are  much  softer  than 
the  crystalline  metamorphic  rocks  in  the  old  Piedmont 
belt  against  which  they  lie.  The  rivers  flowing  from  the 
hard  rocks  of  the  upland  to  the  softer  deposits  of  the 
coastal  plain,  form  falls  or  rapids  at  the  point  of  contact. 
Many    of   the    streams    have   a    deep    navigable    channel 


136 


PHYSICAL  GEOGRAPHY 


across  the  plain  from  the  falls  to  the  sea.  Naturally  these 
falls  were  among  the  first  points  selected  by  the  early  set- 
tlers as  sites  for  their  villages  which  later  grew  into  the 
most  important  cities  along  the  Eastern  United  States. 


Pig.  102.  Fall  line  on  a  small  stream.  The  stream  flows  over 
a  bed  of  hard  limestone  in  the  midst  of  softer  rocks.  The 
falls  visible  in  the  background  are  of  the  Niagara  type  and 
have  receded  about  200  feet  forming  the  gorge  through 
which  the  stream  is  flowing.  The  falls  will  soon  change 
to   rapids.      Why?      (E.    R.    Smith.) 


Besides  the  useful  water  power  obtained  from  the  falls, 
other  reasons  for  their  selection  as  sites  for  cities  were 
the  good  harbors,  connected  by  navigable  water  with  the 
open  sea,  and  a  good  starting  point,  hence  a  good  trading 
point,  for  the  pioneers  who  penetrated  the  interior  of  the 
continent.  The  river  valleys  above  the  falls  even  where 
not  navigable,  furnish  the  best  ways  for  first  the  trail, 
later  the  wagon  road,  and  finally  the  railroad  into  the 


LAKES  137 

interior,  and  the  early  town  thus  grew  into  the  modern 
city. 

Some  of  the  more  important  cities  whose  sites  were 
thus  determined  by  the  waterfalls  of  the  type  mentioned 
are  Philadelphia,  Baltimore,  Washington,  Richmond, 
Raleigh,  Columbia,  and  Augusta.  A  line  drawn  through 
these  cities  is  known  as  the  Fall  Line.*  Trace  out  the 
line  connecting  these  cities  on  a  map  of  the  United  States. 

A  great  many  prosperous  manufacturing  cities  have 
grown  up  inland  around  waterfalls  on  the  streams  because 
of  the  valuable  power  obtainable  for  running  the  ma- 
chinery.-   Let  the  class  make  a  list  of  such  cities. 

REFERENCES 

Lakes : 

Russell,  Lakes  of  North  America.     Ginn  &  Co.,  1895. 

Russell,  Lake  Lahontan,  U.  S.  Geol.  Surv.  Mon.,  XI. 

Gilbert,  Lake  Bonneville,  U.  S.  Geol.  Surv.,  Mon.,  L 

Diller,  Crater  Lake,  Oregon.     Nat.  Geog.  Mag.,  Vol.  8,  p.  33, 
and  U.  S.  Topographic  Atlas. 

Fenneman,  Lakes  of  Southeast  Wisconsin.     Wis.  Geol.  Surv. 
Bull.     8. 

Murdock,  The  Great  Salt  Lake.     Nat.  Geog.  Mag.,  Feb.,  1903, 
p.  75. 
Swamps  and  Marshes: 

Shaler,  6th  An.  Report  U.  S.  Geol.  Surv.  pp.  353-398. 

Shaler,  10th  An.  Report  U.  S.  Geol.  Surv.  pp.  255-339. 
Niagara  Falls,  Grabau,  Bull.     45  N.  Y.  State  Museum,  1901. 
Niagara  Falls,  Gilbert,  Nat.  Geog.  Mon.,  American  Book  Co. 


*  It  is  thought  that  some  of  the  falls  on  this  line  are  due  to  faulting  in  the 
rocks. 


CHAPTER  IV 
GLACIERS 

Switzerland  has  more  visitors  than  any  other  country 
of  equal  area  in  the  world,  due  chiefly  to  its  beautiful 
scenery.  One  of  the  highest  words  of  praise  for  the 
picturesqueness  of  any  portion  of  our  own  country  is  to 
call  it  the  "Switzerland  of  America." 

One  of  the  most  attractive  features  of  Switzerland's 
beautiful  landscapes  is  the  occurrence  of  the  fields  of 
perpetual  snow  on  the  mountains  sending  down  frozen 
rivers  towards,  often  into,  the  green  fields  of  the  valleys. 
These  streams  of  snow  and  ice,  called  glaciers,  begin  in  a 
snow  field  and  end  in  a  river. 

117.  Snow  Fields.— The  temperature  of  the  atmos- 
phere decreases  rapidly  in  ascending  from  the  lower  to 
the  higher  altitudes  and  latitudes.  On  the  very  high 
mountains  in  the  tropics,  on  lower  mountains  in  the  tem- 
perate regions,  and  still  lower  elevations  in  the  polar 
regions,  most  of  the  precipitation  is  in  the  form  of  snow. 
Where  there  is  not  sufficient  warmth  to  melt  all  that  falls, 
there  is  an  accumulation  from  year  to  year  which  forms 
a  snow  field,— the  perpetual  snow  of  the  snow-capped 
mountains. 

Small  snow  fields  occur  on  a  few  of  the  higher  peaks 
of  the  Rocky  Mountains  in  the  United  States;  larger  ones 
on  the  high  peaks  of  the  Sierras;  still  larger  ones  further 
north  along  the  Alaskan  coast;  and  a  much  larger  one 
in  Greenland.  In  fact,  the  whole  area  of  Greenland  ex- 
cept a  narrow  strip  along  the  coast,  is  covered  with  snow 

138 


GLACIERS 


139 


and  ice.     Small  snow  fields  occur  on  the  higher  peaks  in 
Mexico  and  Central  America.     There  are  snow  fields,  large 


Fia.  103.  Aletsch  glacier,  the  longest  glacier  in  Switzerland.  The  dark 
line  in  the  middle  is  the  medial  moraine,  composed  of  rock  fragments 
from  the  distant  mountains.  Notice  the  irregular  surface  caused  by 
the  numerous  crevasses,  and  the  deep  depression  on  each  side  of  the 
glacier.      (Photograph  furnished  by   Colgate   University.) 


140  PHYSICAL  GEOGRAPHY 

and  small,  on  the  higher  mountains  of  the  Alps,  the 
Pyrenees,  in  Scandinavia,  the  Himalayas,  the  Andes,  and 
in  eastern  Africa.  The  largest  snow  field  in  the  world 
at  the  present  time  is  that  on  the  Antarctic  continent. 

There  is  evidence  that  in  the  past  there  were  larger 
snow  fields  than  at  present,  one  of  which  covered  a  large 
area  in  North  America  and  another  a  large  area  in 
Europe. 

From  Snow  to  Ice.  If  the  snow  continued  to  fall  on 
the  upland  regions  faster  than  it  melted  and  there  were  no 
other  escape  for  it,  the  final  result  would  be  to  have  all 
the  water  of  the  oceans,  lakes,  and  rivers  piled  up  around 
the  poles  and  on  the  mountain  tops.  There  is,  however, 
another  escape  for  the  snow.  It  gradually  changes  into 
a  granular  mass  composed  of  little  pellets  of  ice,  resem- 
bling coarse  salt  in  appearance,  known  as  the  neve.  It  is 
similar  to  the  last  remnants  of  snow  drifts  in  the  spring. 
In  the  snow  field  under  the  heat  of  the  sun  and  the  pres- 
sure of  the  mass,  this  granular  snow  or  neve  is  changed 
into  hard  blue  ice,  which  gradually  moves  or  flows  away 
from  the  snow  field.  In  the  continental  snow  fields,  like 
Antarctica,  the  ice  flows  out  in  a  continous  sheet  around 
the  margin,  and  in  the  smaller  snow  fields  on  the  moun- 
tain peaks  the  streams  of  ice  follow  the  deep  valleys,  lead- 
ing down  the  sides  of  the  mountain.  In  some  instances 
there  is  only  one  ice  stream  or  glacier  from  a  snow  field. 
Sometimes  several  glaciers  flow   from  the  same  field. 

There  is  no  sharp  separation  between  a  glacier  and  the  snow 
field  which  is  its  source.  The  ice  that  moves  is  properly  called 
a  glacier,  but  the  bottom  portion  of  the  snow  field  consists  of 
ice,  most  of  it,  probably  all  of  it,  in  motion. 

Many  Alpine  snow  fields  lie  in  amphitheatre-like  basins  called 
cirques,  which  have  been  worn  out  of  the  solid  rock  by  the  ice. 
Fig.  104  shows  several  cirques  formed  in  this  way  from  which 
the  glaciers  have  melted. 


UNIVERSITY 


OF 


Califo^ 


GLACIERS 


141 


Classes  of  Glaciers.  (1)  A  continental  glacier  is  a 
large  field  of  snow  and  ice  that  covers  a  continent  such  as 
Antarctica  or  a  large  part  of  a  continent  such  as  the  one 
that  covered  north  central  Europe  or  central  North  America. 


Fig.  104.  Cirques  or  snow  basins  in  the  Rocky  Mountains  near  Ouray,  Co]. 
(July,  1906.)  1,  a  cirque.  2,  ridge  of  moraine,  composed  of  rock  carried 
out  of  the  basin.  3,  another  smaller  cirque  on  the  same  ridge.  4,  portion 
of  another  cirque  only  the  top  of  which  is  visible. 


(2)  The  Alpine,  or  valley  glacier,  is  the  name  given  to  the 
stream  of  ice  that  flows  down  the  mountain  valleys.  Gla- 
ciers known  as  Piedmont  are  formed  by  the  union  of  several 
valley  glaciers  which  flow  out  on  the  same  plain  and  unite 
into  one.  The  Malaspina  glacier  in  Alaska  is  an  example. 
Cliff  glaciers  form  in  depressions  on  the  mountain  side, 
but  do  not  extend  down  to  the  larger  valleys.  (See  fig.  105.) 
118.  Glacial  Conditions.— Necessary  conditions  for- 
forming  glaciers  are  (1)  heavy  snow  fall,  that  is,  a  greater 


142 


PHYSICAL  GEOGRAPHY 


Fig.  105.  Work  of  a  cliff  glacier  in  the  Rocky  Mountains  near  Ouray,  Col. 
The  darker  band  across  the  Irght  streak,  just  above  the  middle  of  the  view 
is  a  terminal  moraine  of  a  cliff  glacier  that  occupied  the  cirque  above  it. 
The  talus  cone  below  the  moraine  is  composed  of  material  carried  by  gravity 
down  the  mountain  from  the  moraine.  The  loose  material  (2)  above  the 
moraine  is  mostly  talus  formed  by  frost  action  since  the  melting  of  the 
glacier. 

fall  in  the  winter  than  will  melt  in  the  summer,  and 
(2)  a  cool  climate  with  changes  of  temperature.  In  the 
Himalaya  Mountains,  the  glaciers  on  the  south  side,  where 
it  is  warmer,  descend  several  thousand  feet  nearer  sea 
level  than  those  on  the  north  side,  because  of  the  greater 
quantity  of  snow  precipitated  on  the  south  slope.  Changes 
of  temperature  are  needed  both  to  cause  precipitation 
of  moisture  and  to  change  it  to  ice  after  it  has  fallen. 


GLACIERS  .  143 

It  is  warmth  that  causes  the  moisture  first  to  find  its  way 
into  the  atmosphere  from  the  ocean  and  the  moist  land, 
and  second  to  rise  to  the  mountain  top ;  it  is  the  cold  that 
causes  the  precipitation  in  the  form  of  snow,  then  the  heat 
of  the  sun  changes  the  snow  first  to  neve  and  finally  to  ice 
and  causes  it  to  flow  down  the  mountain. 

119.  Movements  of  the  Glacier.— It  was  a  long  time 
after  the  existence  of  glaciers  was  known,  before  it  was 
understood  that  the  ice  really  moved.  Even  at  the  pres- 
ent time  there  is  some  uncertainty  as  to  just  why  and  how 
it  moves,  but  the  fact  that  it  does  move  is  proven  beyond 
question.  After  the  study  of  this  chapter,  the  pupil 
should  enumerate  all  the  points  of  evidence  that  the  gla- 
ciers move. 

The  rate  of  movement  varies  in  different  glaciers  and 
even  in  the  same  glacier  at  different  seasons.  The  move- 
ment is  generally  faster  in  a  large  glacier  than  in  a  small 
one  under  the  same  conditions.  It  is  faster  in  a  warm 
season  than  in  a  cold,  faster  in  the  middle  than  at  the 
sides,  faster  at  the  top  than  on  the  bottom,  faster  on  the 
outside  of  a  curve  than  on  the  inside.,  faster  on  the 
steeper  slopes.     Why? 

One  of  the  ways  in  which  the  rate  of  movement  of 
different  portions  of  a  glacier  was  determined  was  to  put 
a  row  of  stakes  across  the  ice  in  line  with  one  on  the  rocks 
on  each  side.  After  a  few  months  the  straight  line  had 
the  position  shown  in  fig.  106. 

In  1821,  two  men  were  lost  in  a  crevasse  on  the  Bossons 
glacier  on  Mt.  Blanc.  Professor  Forbes,  who  had  been  study- 
ing that  glacier  made  the  statement  that  the  remains  of  these 
men  would  appear  at  the  lower  end  of  the  glacier  in  40  years, 
the  length  of  time  it  would  take  the  glacier  to  move  from  the 
crevasse  to  its  lowest  point,  a  distance  of  about  one  mile.  In 
1861,  when  the  mangled  remains  of  these  men,  and  some  of 
the   instruments   carried   by  them   appeared  at  the   end  of  the 


144 


PHYSICAL  GEOGRAPHY 


glacier,    the    forecast   of  Forbes 
veled  at  its  accuracy. 


was   recalled   and   people   mar- 


120.  Variation  in  Length  of  a  Glacier.— The  snow  line 
is  the  elevation  on  the  mountains  above  which  the  snow 
lies  all  the  year,  and  below  which  it  is  all  melted  during 
the  summer  season.     The  glacier  must  originate  above  the 


B 


e     •     «    e 


Fia.  106.  Diagram  indicating  one  of  the  ways  in  which  movement  of  the  ice 
in  glaciers  was  demonstrated.  A  line  of  stakes  was  extended  across  a 
glacier  continuous  with  those  on  the  bordering  mountain  side.  At  suc- 
cessive intervals  the  stakes  were  observed  in  the  positions  shown.  B 
shows  the  changes  in  a  row  of  stakes  on  the  vertical  side  of  a  glacier. 


snow  line  and  as  soon  as  it  crosses  that  line  it  begins  to 
waste  from  melting.  The  distance  it  descends  below  the 
snow  line  depends  upon  the  size  of  the  glacier,  its  rate  of 
movement,  and  the  climate  in  which  it  moves.  It  will 
melt  nearer  the  snow  line  in  the  tropics  and  descend 
further  below  the  snow  line  in  the  temperate  and  frigid 
climates. 

It  is  only  in  high  latitudes  that  the  glacier  descends 
as  low  as  sea  level.  In  temperate  and  tropical  regions  it 
descends  until  the  rate  of  melting  just  equals  the  rate  of 
advance.     At  this  point,  which  is  the  end  or  terminus  of 


GLACIERS 


145 


the  glacier,  the  ice  is  still  moving,  but  it  is  melted  as  fast 
as  it  moves  and  the  end  of  the  glacier  remains  fixed  as 
long  as  the  conditions  remain  constant. 

One  or  more  warm,  dry  seasons  in  which  there  is  less  snow- 
fall and  more  melting,  will  cause  the  end  of  the  glacier  to  move 
back  up  the  valley  and  establish  a  new  point  of  equilibrium  in 
accord  with  the  new  conditions.  In  the  same  way  one  or  more 
cool,  wet  seasons  in  which  there  is  unusually  heavy  snowfall 
and  less  rapid  melting,  will  cause  the  end  of  the  glacier  to  ad- 
vance down  the  valley  a  greater  distance  until  conditions  are 
again  balanced.  In  this  way  the  glacier  serves  as  a  good  indi- 
cator of  climatic  variations.  With  one  or  two  exceptions  all 
the  glaciers  of  the  Alps  are  now  retreating.  In  the  early  part 
of  the  last  century  they  were  advancing. 

121.     Crevasses. — In  the  steeper  parts   of  the  chan- 


Fia.  107.  Snowfield  and  crevasses,  Alpine  glacier.  Some  of  the  boulders  on 
the  surface  fall  to  the  bottom  through  the  crevasses.  Fresh  snow  some- 
times driffs  over  and  closes  the  top  of  a  crevasse  thus  forming  a  death 
trap  for  the  unwary  traveler  who  steps  upon  the  snow  and  falls  through 
into  the  depths  of  the  glacier.  (Colgate  University.) 
10 


146  PHYSICAL  GEOGRAPHY 

nel  which  would  correspond  to  the  rapids  and  waterfalls 
in  the  river,  the  glacier  is  very  uneven,  with  many  deep 
cracks  or  fissures  called  crevasses.  Below  the  rapids  these 
close  up  in  part  by  regelation  or  refreezing  and  the  ice 
stream  passes  on,  as  above  the  rapids.  The  crevasses  are 
formed  wherever  there  is  a  change  in  the  angle  of  slope 
of  the  channel  in  which  it  is  moving,  and  on  the  outside 
of  the  curve,  where  there  is  a  bend  in  the  valley.  (Fig.  107.) 

122.  Ice  Tables  and  Pinnacles.— A  large  flat  rock  on 
the  surface  of  the  glacier  protects  the  ice  underneath  from 
the  sun's  rays  so  that  it  does  not  melt  as  rapidly  as  the 
surrounding  ice,  the  result  being  that  the  rock  is  finally 
left  standing  on  a  column  of  ice  like  a  pyramid.  When 
the  surrounding  ice  is  melted  away  several  feet  below  the 
rock,  the  sun  shines  underneath  the  capstone,  melting  the 
ice  on  that  side  and  the  rock  falls  off,  leaving  a  pinnacle 
or  needle  of  ice.  (Will  the  cap  rocks  fall  on  the  north 
or  south  side  of  the  pinnacle  in  the  northern  hemisphere?) 

The  ice  tables  are  more  apt  to  occur  near  the  lower  end  of 
the  glacier.  Why?  They  are  more  common  in  a  dry,  hot  sum- 
mer  than  a   cold   one.     Why? 

123.  Other  Surface  Irregularities.— In  a  similar 
manner  the  medial  moraine  protects  the  ice  from  melting 
underneath  it;  so  that  it  frequently  forms  a  prominent 
ridge  of  rock  fragments  along  the  middle  of  the  glacier. 
(See  fig.  103.) 

The  Alpine  or  valley  glaciers,  below  the  snow  line,  flow  be- 
tween rock  walls  that  are  bare  of  snow  in  the  summer  season. 
The  heat,  absorbed  and  radiated  on  the  glacier  from  the  rocks 
where  they  receive  the  direct  sunlight,  causes  the  glacier  to 
melt  more  rapidly  on  the  side  than  in  the  middle.  Hence,  in 
crossing  a  glacier  in  such  places  one  must  ascend  a  hill  of  ice 
before  reaching  the  middle  of  the  glacier.  (See  lower  end  of 
Aletsch  glacier  fig.  103.) 


GLACIERS  147 

In  the  narrower  canyons,  where  the  glacier  and  the  rocks 
bordering  it  do  not  receive  the  direct  rays  of  the  sun,  the  ice 
may  be  as  high  or  higher  at  the  sid6  than  in  the  middle  of 
the  glacier. 

Small  stones  and  thin  patches  of  dust,  instead  of  forming 
pinnacles  like  the  large  boulders,  sink  into  the  ice  and  make 
little  holes  called  diist  wells  that  are  filled  with  water  which 
freezes  over  at  night  and  thaws  during  the  day. 

124.  Moraines.— As  the  glacier  moves  over  the  sur- 
face, it  scrapes  off  large  quantities  of  soil  and  even 
some  fresh  rock,  especially  where  the  underlying  rock 
projects  above  the  surface  in  sharp  points  or  ledges.  Be- 
sides the  material  pushed  and  shoved  along  in  front  of 
and  underneath  the  ice  there  is  a  considerable  quantity 
frozen  in  the  lower  portions  of  the  ice.  "Where  the 
glacier  flows  along  the  base  of  a  cliff,  it  receives  all  the 
rock  fragments  that  fall  from  the  cliff  through  the  action 
of  frost,  gravity,  and  other  weathering  agencies.  As  the 
ice  is  advancing  slowly  all  the  time,  the  material  is  mov- 
ing forward,  forming  a  band  of  rocky  material  along  the 
side  of  the  glacier  below  the  cliff,  known  as  the  lateral 
moraine.  Where  two  glaciers  unite,  the  lateral  moraines 
at  the  point  of  junction  will  unite  in  the  midst  of  the 
combined  glacier  and  form  a  medial  or  middle  moraine. 
(See  fig.  103).  Where  there  are  many  tributaries,  there 
may  be  many  lines  of  medial  moraines. 

In  some  instances  the  moraines  are  so  numerous  and 
so  large  as  to  entirely  cover  the  surface  of  the  ice,  as  is 
the  case  of  the  Unter  Aar  glacier  in  Switzerland.  The 
material  carried  in  the  bottom  and  underneath  the  ice 
forms  the  ground  moraine.  Medial  moraines  may  some- 
times be  formed  of  material  from  the  ground  moraine  or 
material  underneath  the  glacier  which  is  carried  upward 
through  the  glacier  by  an  upward  current  in  the  ice  until 


148  PHYSICAL  GEOGRAPHY 

it  reaches  the  surface,  forming  a  medial  moraine  or  in- 
creasing the  size  of  the  one  already  formed. 

All  the  rock  material  carried  by  the  ice,  the  medial, 
lateral  and  ground  moraines,  is  dropped  at  the  end  of  the 
glacier  and  there  forms  the  terminal  moraine.  During 
the  retreat  of  the  glacier  if  the  end  remains  stationary 
for  a  period  of  time,  the  material  accumulates  as  in  the 
first  terminal  moraine,  from  which  it  may  be  distinguished 
by  calling  it  a  recessional  moraine  or  a  terminal  moraine 
of  recession.     (See  fig.  105.) 

125.     Special  Topographic  Forms  of  Glacial  Deposit.— 


Fig.  108.  Till  or  boulder  clay  on  Fayette  Street,  Syracuse,  N.  Y. 
It  consists  of  a  mass  of  tough  clay  interspersed  with  numer- 
ous partially  rounded,  striated  boulders.  The  entire  mass 
was  imbedded  in  the  bottom  of  the  ice  or  pushed  along 
underneath  the  glacier. 

All  of  the  material  carried  by  the  glacier  is  deposited 
somewhere,  some  of  it  at  the  end,  some  of  it  underneath 
the  ice.  That  deposited  at  the  end  of  the  glacier,  while 
it  is  stationary  or  nearly  so,  forms  the  terminal  moraine 


GLACIERS  149 

which  generally  consists  of  irregular  hills  and  ridges. 
The  moraine  in  some  places  consists  of  a  single  ridge, 
while  in  others  it  occupies  an  area  covered  irregularly  with 
hills  of  different  sizes,  sometimes  inclosing  basin-like  de- 
pressions called  kettle  holes.  When  these  are  filled  with 
water  they  are  called  kettle  lakes  and  the  whole  area 
called  the  kettle  moraine.  Where  the  glacier  retreats 
regularly,  the  moraine  material  is  distributed  somewhat 
uniformly  over  the  area. 

Drift  is  the  name  given  to  all  the  material  deposited 
by  a  glacier  when  it  disappears  by  melting. 

Till,  or  boulder  clay,  is  the  unsorted  material  of  the 
ground  moraine  consisting  of  clay,  frequently  blue  clay, 
interspersed  with  more  or  less  sand  and  boulders,  the  latter 
frequently  striated  and  facetted  by  being  rubbed  against 
other  pebbles  or  against  the  bed  rock.     (See  fig.  120a.) 

A  kame  is  a  low  hill  of  gravel  and  sand,  partly  but 
irregularly    stratified,    commonly   elongated    transverse   to 


Fig.  109.  Kame  topography.  Mendon,  Monroe  Co.,  N.  Y.  Karnes  are 
composed  of  gravel  and  sand  in  irregular  ridges,  commonly  formed  in 
crevasses  near  the  margin  of  the  glacier.      (H.  L.  Fairchild.) 

the  direction  of  the  ice  movement.     It  is  thought  to  have 
formed  in  cracks  or  crevices  in  the  ice  near  the  end  or 


150  PHYSICAL  GEOGRAPHY 

margin  of  the  glacier,  from  materials  washed  in  by  sur- 
face streams  and  the  waters  from  the  melting  ice  and 
rains. 

An  esker  is  a  low  winding  ridge  of  sand  and  gravel 
generally  elongated  in  the  direction  of  the  ice  movement. 


Fig.  110.  A  small  esker,  near  Jamesville,  N.  Y.  Probably  formed  by  the 
accumulation  of  gravel  and  sand  in  a  stream  channel  underneath  the 
ice. 

It  may  vary  from  a  fraction  of  a  mile  to  many  miles 
in  length  and  is  probably  formed  by  the  accumula- 
tions in  a  sub-glacial  stream  which  at  the  time  would  have 
sides  and  roof  of  ice.  The  esker  is  in  general  longer, 
lower,  more  winding  and  extended  in  a  different  direction 
from  the  kame.    (See  fig.  110.) 

A  drumlin  is  a  rounded  egg-shaped  hill  composed  of 
till  or  unstratified  drift,  commonly  elongated  in  the  direc- 
tion of  ice  movement  and  usually  about  200  or  250  feet  in 


GLACIERS  151 

height,  sometimes  lower  but  rarely  higher  than  that  above 
the  base.  Frequently  there  are  pockets  of  sand  and 
gravel  in  the  surface  of  the  drumlin,  although  the  greater 
part    of    the    outside    consists    of    boulder    clay.      It    is 


Pig.  111.  Drumlin  on  Euclid  Avenue,  Sjrracuse,  N.  Y.,  looking  east. 
The  hill  is  composed  of  boulder  clay  deposited  underneath  the  ice. 
In  this  case  the  north  end  is  much  steeper  than  the  south  end  but  in 
some  drumlins  the  two  ends   are   similar. 

probable  that  the  central  core  of  many  drumlins  is  solid 
rock.  Excavations  either  natural  or  artificial  into  the  in- 
terior are  so  few  in  number  that  there  is  some  uncertainty 
about  drawing  general  conclusions  regarding  their  inter- 
nal structure.  Drumlins  are  formed  underneath  the 
glacier  by  the  heaping  up  of  the  ground  moraine.  There 
are  scores  of  drumlins  along  the  line  of  the  Erie  Canal 
through  central  New  York  from  east  of  Syracuse  to  west 


152 


PHYSICAL  GEOGRAPHY 


of    Rochester.      They    are    abundant    in    eastern    Massa- 
chusetts, in  Wisconsin  and  Michigan. 

126.  Corrading  Work  of  Glaciers.— Glaciers  are  and 
have  been  important  geologic  agents  both  in  grinding  the 
rock  surfaces  and  in  transporting  material.  In  corrading 
or   grinding   the   surface   rocks,   the   glacier   acts   like    a 


Fig.  112.  Glacial  scratches  (striae)  on  sandstone  on  tho 
Catskill  Mountains  at  an  elevation  of  2,400  feet.  (H.  D. 
McGlashan.) 


coarse  sandpaper  pushed  over  the  area.  The  continental 
glacier  that  covered  the  Northern  United  States  must  have 
been  several  thousand  feet  thick  in  many  places  and  pushed 
its  rough-shod  surface  against  the  rocks  with  tremendous 
force. 

Ice  weighs  over  50  pounds  per  cubic  foot,  hence  a 
glacier  5,000  feet  thick  would  press  down  with  a  force  of 
more  than  250,000  pounds  on  every  square  foot,  but  in 
many  places  the  ice  was  10,000  feet  or  more  in  thickness, 


GLACIERS 


153 


hence  its  corrading  effect  on  the  rock  surface  over  which 
it  moved  must  have  been  very  great. 

The  corrading  action  of  the  glacier  is  indicated  by  the 
scratching,  grooving,  and  polishing  of  the  rocks  of  all  kinds  over 
which  it  passed,  and  by  the.  vast  quantities  of  ground-up  fresh 
rock  which  it  deposited  along  its  course.  Pulverized  soil  and 
mantle  rock  are  prevailingly  yellow,  brown  or  red  in  color,  but 


Fig.  113.  Glacial  grooves  in  volcanic  rock  on  the  side  of 
Uncompaghre  Canyon,  near  Ouray,  Col.  Extensive  areas 
in  this  canyon  are  bare  of  any  fragmental  material  and  in 
many  places  show  the   abrading  action  of  the  glacier. 

ground-up  fresh  rock  is  blue.  Besides  the  scratching  and 
grooving  of  the  bed  rock,  many  of  the  loose  boulders  in  the 
glacial  deposit  are  striated,  grooved,  and  partly  rounded  by 
being  rubbed  against  other  rocks,  (Where  possible,  study  the 
rock  surface  over  which  the  ice  has  passed  and  carefully  com- 
pare boulders  from  the  glacial  deposit  with  those  from  a 
stream  channel,  or  from  a  sea  or  lake  beach.)  (Fig.  120a  p.  161.) 
The  grinding  action  was  of  course  not  uniform  over  the 
entire  area.  In  a  hilly  region  the  tendency  would  be  to  wear 
away  projections  and  make  a  smoother  surface.  The  valley 
glaciers  and  the  ice  tongues  of  the  continental  glaciers  extend- 


154 


PHYSICAL  GEOGRAPHY 


ing  in  valleys,  deepen  and  widen  the  valley,  making  a  U-shaped 
glacial  valley  out  of  the  V-shaped  stream  valley  (fig.  114).  How- 
ever, in  passing  across  valleys  transverse  to  the  direction  of 
movement,  the  tendency  would  be  to  deposit  material  in  the 
bottom  of  the  valley  and  wear  away  the  top  of  the  hill,  thus 
making  the  surface  more  regular  than  it  was  before.  We 
might  expect  also  that  in  passing  over  a  hill,  a  glacier  would 


Fig.  114.  A  U-shaped  glaciated  valley  near  Ouray,  Col.  Much  of  the 
material  from  this  valley  was  eroded  by  the  glacier.  Glacial  cirque 
visible  in  background.  There  are  three  cirques  at  the  head  of  the 
valley  from  which  the  united  glaciers  extended  down  the  U-shaped 
valley  in  the  foreground. 


corrade  more  on  the  stoss  or  thrust  side  of  the  hill  (the  side 
with  which  it  first  came  in  contact)  than  on  the  lee  side  where 
it  might  even  deposit  material.  In  places,  basin-shaped  depres- 
sions are  scooped  out  in  the  rock  which  on  the  retreat  of  the 
glacier  are  filled  with  water  and  become  lakes.  (See  the  Silver- 
ton  and  Telluride  quadrangles  of  the  U.  S.  Topographic  Atlas 
where  scores  of  these  cirque  lakes  are  shown.  Fig.  115  is  a  view 
in  the  northwest  corner  of  the  Silverton  Quadrangle.  See  also 
Chap.  III.) 

In  many  places  in  a  flat  region  the  glaciei  moves  over  a  bed 


GLACIERS 


155 


of  sand  or  clay  with  almost  no  corrading  action.  In  fact,  in 
many  places  the  corrasion  is  just  as  much  on  the  hard  rock  as 
on  the  soft  material  and  sometimes  more.  In  this  respect  it  is 
markedly  different  from  the  corrading  action  of  a  river,  which 
always  attacks  the  softest  material  first. 


fkiJ 

•   ^« 

Fig.  115.  Silver  Lake  Basin,  near  Ouray,  Col.  The  lake  in  the  foreground 
is  formed  behind  a  moraine.  The  low  dark  ridge  in  the  middle  of  the 
picture  is  another  moraine  behind  which  is  another  lake  similar  to  the 
one  in  the  foreground.     The  whole  basin  including  both  lakes  is  a  cirque. 


Over  hard  massive  rocks  the  glacier  frequently  erodes 
the  surface  in  small  rounded  domes  which  at  a  distance  re- 
semble the  backs  of  sheep  and  are  called  roches  moutonnees 
(rock  sheep).  This  type  of  glacial  erosion  is  common  on 
the  crystalline  rocks  of  the  Canadian  highlands  and  on  the 
granite  and  volcanic  rocks  of  the  Rocky  and  Sierra  Moun- 
tains. 

Huge  pot  holes,  sometimes  20  feet  or  more  in  depth  are 
formed  in  places  underneath  the  glacier  by  the  grinding  of 
the  boulders  where  they  were  whirled  about  by  streams  de- 


156 


PHYSICAL  GEOGRAPHY 


scending  through  a  crevasse.  (See  fig.  116.)  Glacial  pot 
holes  are  similar  in  some  respects  to  those  formed  by 
streams  on  the  rapids.     (See  sec.  72.) 


Fig.  116.  Pot  holes  on  grooved  and  striated  rock  surface  in  Glacier  Garden, 
Lucerne,  Switzerland.  Larger  pot  holes  are  shown  elsewhere  in  the  same 
garden.  They  were  formed  by  streams  falling  through  the  ice  when  the 
glacier   covered  the   area,      (J.   C.    Branner.) 

Hanging  Valleys.— The  glacier  sometimes  erodes  the 
main  valley  much  deeper  than  the  tributary  valleys  and 
when  the  glaciers  are  melted  from  the  area,  the  streams  of 
water  in  the  tributary  valleys  enter  the  main  valley  over 
a  cliff,  forming  cataracts.  Such  tributaries  are  known  as 
hanging  valleys  and  are  common  in  many  glaciated  areas. 

127.  Glaciers  as  Transporting  Agents.— The  glaciers 
are  probably  more  important  transporting  agents  than 
corrading  ones,  since  they  not  only  carry  all  the  material 
they  wear  off  the  rocks  over  which  they  pass,  but  large 
quantities  that  gain  access  to  the  glacier  in  other  ways. 


GLACIERS  157 

It  carries  material  frozen  in  its  under  surface  and  pushed 
underneath  and  in  front,  suhglacial  material;  material  on 
the  surface  that  falls  from  cliffs  and  mountains  by  which 


Fig.  117.  Glacial  Hanging  Valley  in  Norway.  The  streams 
from  the  tributary  valleys  descend  by  a  series  of  cascades 
to  the  floor  of  the  main  valley  which  was  worn  down  by  the 
glacier. 

it  passes,  and  that  swept  on  its  surface  by  avalanches  and 
otherwise— super  glacial  material;  and  that  in  the  midst 
of  the  ice  between  the  top  and  the  bottom,  englacial  ma- 
terial. 

128.  The  Materials  Carried  by  Glaciers.— The  glac- 
iers carry  all  sizes  of  material  from  the  very  fine  to  the 
very  coarse.  The  coarse  and  fine  are  sometimes  sorted 
and  separated  by  the  waters  associated  with  the  melting 
ice,  but  frequently  they  are  deposited  in  a  heterogeneous 
mass.  The  bulk  of  the  material  in  most  glacial  deposits 
over  northern  United  States  will  be  found  on  inspection 
to  consist  of  rock  materials  that  have  come  from  an  area, 
much  of  it  within  one  or  two  miles  of  the  place  where  it 


158 


PHYSICAL  GEOGRAPHY 


is  deposited.  A  part  of  the  deposit,  however,  has  been 
transported  for  a  long  distance,  sometimes  a  hundred 
miles  or  more.     Through  central  and  southern  New  York, 


Fig.  118.  A  crystalline  glacial  boulder  transported  by  the  glacier  from 
Canada  to  Syracuse,  N.  Y.  Shown  as  it  was  being  moved  by  man  to 
the  cemetery  to  serve  as  a  monument. 

there  are  a  great  many  boulders,  large  and  small,  that 
have  been  moved  by  the  glacier  from  the  Canadian  high- 
lands north  of  Lake  Ontario.  One  of  these,  known  as  the 
Grouse  boulder,  now  in  the  cemetery  at  Syracuse,  weighs 
about  75  tons,  and  was  carried  by  the  glacier  from  Canada 
to  Central  New  York. 

The  large  boulders  are  mostly  carried  on  or  in  the 
glacier  and  when  the  ice  melts,  the  boulder  is  sometimes 
left  on  a  very  insecure  foundation.  In  such  positions 
they  are  called  perched  boulders  or  rocking  stones.  (Fig. 
119).     Boulders  of  disintegration  that  sometimes  resemble 


GLACIEES 


159 


glacial  rocking  stones  may  be  distinguished  by  noting  that 
they  are  the  same  kind  of  rock  as  that  on  which  they  are 
perched.  Fig.  120  shows  a  boulder  of  disintegration  in 
the  Garden  of  the  Gods. 


Fig.   119.     Perched  boulder  or  rocking  stone  deposited  by  the  glacier  near 
Greensboro,  Vt.      (C.  H.  Richardson.) 


The  material  transported  by  a  glacier  is  deposited  when  the 
ice  melts.  As  the  greatest  melting  takes  place  at  the  lower  end 
of  the  glacier,  there  the  greatest  deposits  will  be  formed.  In 
some  places  the  material  is  spread  somewhat  evenly  over  the 
surface  but  frequently  it  is  deposited  in  the  form  of  drumlins, 
kames,  eskers  or  the  irregular  mounds  of  a  terminal  moraine. 

The  drift-covered  surface  of  a  region  that  has  been  covered 
by  a  continental  glacier  is  quite  different  from  the  surface  of  the 
bed  rock  on  which  the  drift  material  rests,  and  both  the  surface 
of  the  drift  and  the  surface  of  the  bed  rock  are  different  from 


160 


PHYSICAL  GEOGRAPHY 


that  of  the  same  area  before  the  glacier  passed  over  it.  Hence, 
the  topography  of  a  glaciated  region  is  characteristic  and  dis- 
tinct from  that  of  an  unglaciated  region. 


Fig.  120.  Boulder  of  disintegration  in  the  Garden  of  the  Gods,  Colorado. 
It  is  the  remnant  of  a  bed  of  sandstone;  the  surrounding  portions 
have  been  carried  away  by  rain  and  winds.  It  has  not  been  trans- 
ported from  another  locality  like  the  two  preceding  ones.  (G.  H. 
Ashley. ) 

129.  Icebergs.— The  ice  composing  the  glacier  is  dis- 
posed of  in  two  ways.  In  case  of  the  large  glaciers  in 
high  latitudes,  the  ice  stream  flows  into  the  sea  and  moves 
out  into  the  salt  water  until  the  end  is  broken  off  and 
floats  away  as  a  large  cake  of  ice,  called  an  iceberg.     By 


GLACIERS 


161 


Fia.  120a.     Boulders  of  different  origin.      1  and  2  glacial  boulders. 

and  5,  boulders  worn  and  polished  by  wind-blown  Sand  on  the  desert. 
6,  boulder  of  disintegration.     7-11,  boulders  from  a  rocky  beach. 

the  action  of  ocean  currents,  icebergs  slowly  drift  towards 
the  equator,  gradually  melting  as  they  move,  until  they 
finally  disappear.     The  largest  icebergs  in  the   southern 


yiG.  121,      Diagram  illustrating  how  icebergs  are  formed  where  glaciers  flow  in- 
to the  sea.      The  end  of  the  glacier  in  the  sea  is  raised  and  lowered  by  the 
tides  and  waves  until  it  breaks  off  and  floats  away  in  an  iceberg. 
11 


162  PHYSICAL  GEOGRAPHY 

hemisphere  come  from  the  polar  ice  cap  which  covers  the 
Antarctic  continent.  In  the  Northern  hemisphere  the 
largest  ones  come  from  the  coast  of  Greenland. 

130.  Melting  of  Glaciers.— Outside  the  polar  regions 
glacial  ice  is  disposed  of  by  melting  on  the  land  before  it 
reaches  the  sea.  Melting  goes  on  along  the  whole  length 
of  the  glacier  in  the  warm  season,  but  is  most  active  at 
the  lower  end  where  the  air  is  denser  and  warmer.  The 
water  melting  on  the  surface  of  the  glacier  forms  streams 
which  flow  along  the  top  until  they  come  to  open  cracks 
or  crevasses  through  which  they  drop  to  the  bottom. 

In  the  Alpine  or  valley  glaciers,  the  subglacial  streams  gen- 
erally unite  under  the  ice  and  emerge  from  the  end  of  the 
glacier  in  a  single  stream  which  frequently  flows  from  an  ice 
cave. 

In  the  case  •of  a  continental  glacier  there  will  be  many 
streams   flowing   from    the   margin    of   the   glacier.     Where    the 


FiCJ.  122.  View  in  une  of  the  cross  glacial  channels,  near 
Manlius,  N.  Y.  The  level  floor  of  the  channel,  an  eighth 
of  a  mile  in  width,  is  bordered  by  limestone  cliflfs  about  200 
feet  high.     The  head  of  the  channel  is  visible  in  the  distance. 

land  slopes  away  from  the  glacier,  streams  will  run  off  through 
every  depression  or  valley,  and  the  vast  quantities  of  rock  ma- 


GLACIERS  163 

terial  swept  along  by  these  streams  is  distributed  down  the 
valleys  beyond  the  margin  of  the  glacier  and  known  as  valley 
trains.  In  the  absence  of  valleys,  it  will  be  spread  out  over  the 
plain  beyond  the  glacier,  forming  a  glacial  apron  or  an  overwash 
plain. 

131.  Glacial  Channels.— Where  the  land  slopes  in  the  direc- 
tion from  which  the  glacier  is  moving,  there  will  be  an  accu- 
mulation of  water  in  the  depression  at  the  end  of  the  ice,  form- 


:.:mf 

Fig.  123.  View  at  the  head  of  the  channel  shown  in  Fia.  122. 
The  former  vertical  cliff  has  been  partly  eroded  at  the  top 
and  the  falls  are  changing  to  rapids. 

ing  a  lake.  The  water  from  the  melting  ice  here  mingles  with 
the  water  draining  from  the  land,  until  it  fills  the  depression 
and  overflows  at  the  lowest  point.  In  central  New  York, 
where  the  glacier  was  moving  south  and  the  rivers  flowing 
north,  the  ice  formed  a  dam  across  all  the  north  draining  val- 
leys which  then  fllled  with  water  up  to  the  lowest  point  in 
the  divide,  when  the  water  overflowed  and  often  cut  a  deep 
channel  through  into  the  next  valley.  Where  the  water  in  one 
of  these  cross  channels  flowed  over  a  ledge  of  hard  rock  on  to  a 
softer  layer,  waterfalls  like  Niagara  were  formed.  Some  of  the 
streams  near  Syracuse  were  probably  as  large  or  even  larger 
than  the  Niagara  River.    On  the  further  retreat  of  the  glacier 


164  PHYSICAL  GEOGRAPHY 

the    streams    disappeared   but   the    pools    at   the   bottom    of   the 
waterfalls   remained  as   lakes  in  the  area  mentioned. 

There  are  scores  of  these  east  and  west  cross-valleys  formed 
in  this  way  across  the  divides  between  the  north-flowing  streams 
in  central  New  York.  (For  good  examples  of  these  cross  chan- 
nels study  the  Syracuse,  Tully  and  Skaneateles  sheets  of  the 
contour  map.  See  also  maps,  diagrams,  and  descriptions  of 
these  channels  by  Professor  Fairchild  in  the  21st  Annual  Report 
of  the  State  Geologist  of  New  York.)   (Figs.  122  and  123). 

132.  Glaciers  Compared  with  Rivers.— In  some  ways 
glaciers  are  like  rivers;  in  others  they  are  very  much  un- 
like them.  Alpine  or  valley  glaciers  resemble  rivers  in 
flowing  through  valleys  or  elongated  depressions  in  the 
surface  and  along  the  lowest  part  of  the  valley ;  in  having 
crooks  and  turns,  and  falls  and  rapids;  in  moving  faster 
at  the  top  than  at  the  bottom,  faster  on  the  outside  of  a 
curve  than  on  the  inside;  in  moving  more  rapidly  and 
having  a  rougher  surface  on  the  steeper  portions  of  the 
channel,  namely  the  falls  and  rapids,  than  on  the  level 
portions;  in  being  fed  by  moisture  precipitated  from  the 
atmosphere;  in  carrying  this  precipitated  moisture  to  or 
towards  the  sea  level;  in  carrying  vast  quantities  of  rock 
material  from  higher  to  lower  levels. 

Glaciers  differ  from  rivers  in  moving  much  more  slow- 
ly—inches per  day  instead  of  miles.  Both  are  fed  by 
rains  and  snow,  but  rivers  are  fed  chiefly  by  rain  and 
groundwater,  while  glaciers  are  fed  almost  entirely  by 
snow.  The  source  of  a  glacier  is  always  a  snow  field;  of 
a  river  it  is  springs  or  a  spring,  a  lake,  or  sometimes  even 
a  glacier.  Glaciers  carry  more  and  coarser  material  on 
the  surface  than  a  river,  which  carries  all  its  coarse  ma- 
terial by  rolling  and  pushing  it  along  the  bottom. 

Rivers  may  carry  heavy  loads  down  steep  slopes,  but 
they  drop  the  greater  part  of  the  load  on  the  first  flat, 
while  glaciers  carry  heavy  burdens  over  flats  and  in  many 


GLACIERS  165 

cases  even  up  hill.  The  river  carries  most  of  its  burden 
during  the  flood  season,  dropping  much  of  it  with  the  sub- 
sidence of  the  flood,  while  the  glacier  carries  its  burden 
steadily  along  until  it  drops  it  at  the  end  of  the  ice  or 
until  it  becomes  lodged  underneath  the  ice.  Rivers  wear 
away  first  the  softer  parts  of  the  rock  over  which  they 
flow,  while  glaciers  wear  away  both  hard  and  soft.  Gla- 
ciers form  moraines,  rivers  form  flood  plains  and  deltas. 
Rivers  are  frequently  very  important  highways  of  com- 
merce, while  glaciers  are  obstructions.  In  what  other 
ways  do  glaciers  and  rivers  resemble  each  other  and  differ 
from  each  other  ? 

Glaciers  form  an  important  part  in  the  freshwater  circula- 
tion of  the  globe — a  frozen  portion  of  the  circle  that  checks  and 
retards  the  rate  but  fortunately  does  not  fc:top  it  entirely. 

The    North    American    Continental    Glacier.— In    the 

geological  age  immediately  preceding  the  present,  a  large 
part  of  North  America  was  covered  with  a  great  fleld  of 
snow  and  ice.  There  were  three  centers  of  accumulation 
from  which  the  ice  moved  out  radially;  one  called  the 
Labrador  center  was  east  of  Hudson  Bay,  another  the 
Keewatin  center  was  directly  west  of  Hudson  Bay  and 
another  known  as  the  Cordilleran  was  in  Western  Canada. 
From  these  three  centers  the  ice  spread  out  until  it 
covered  nearly  all  of  Canada  and  a  large  part  of  northern 
United  States.  Many  of  the  geographical  features  of  this 
area  are  due  directly  or  indirectly  to  the  action  of  this 
now  extinct  glacier.  It  deepened  and  widened  some  of 
the  valleys,  it  filled  and  destroyed  others,  it  caused  the 
shifting  of  many  river  channels.  It  changed  the  form  of 
many  of  the  existing  hills  and  formed  some  additional 
ones.  It  scraped  mantle  rock  from  some  places  and  de- 
posited it  in  others. 


166 


PHYSICAL  GEOGRAPHY 


What  are  some  of  the  other  effects  produced  by  the  North 
American  Glacier?  Try  to  picture  in  your  mind  some  of  the 
results  from  the  movement  of  such  a  great  sea  of  ice  into  this 


Fig.  124.  Map  of  North  America — the  ice  age,  showing  the  part  covered  by 
the  ice  and  the  three  centers  of  accumulation  from  which  the  ice  moved. 
(After  Chamberlin. ) 


country  now;  its  effect  on  the  streams,  lakes,  hills,  soil,  vegeta- 
tion, animals  and  man;  the  conditions  during  the  advance  and 
those  during  the  retreat  or  melting  of  the  glacier.    The  condi- 


GLACIERS  167 

tions  several  thousands  of  years  after  the  melting  are  those  we 
have  at  the  present  time. 

To  produce  this  great  glacier  there  was  of  course  a  change 
in  the  climate,  in  fact,  two  changes,  one  an  increase  in  cold  to 
produce  the  ice  and  second  a  warmer  change  to  cause  its  melt- 
ing and  disappearance.  The  probable  causes  for  these  changes 
is  a  topic  too  large  for  discussion  here.  One  cause  was  the 
elevation  of  the  area  to  higher  altitudes.  Another  probable 
cause  was  the  variation  of  the  amount  of  carbon  dioxide  in  the 
atmosphere.  (For  good  discussion  see  Geology  by  Chamberlin 
and  Salisbury,  page  424.) 

133.    The  Economic  Effects  of  Glaciation.— After  the 

study  of  glacial  phenomena  in  the  preceding  pages  the 
reader  should  draw  his  own  inferences  as  to  the  effect  of 
the  glacier  on  the  industries  of  man,  on  the  area  of  the 
northern  United  States.  The  present  soil  is  markedly 
different  from  that  before  the  passage  of  the  glacier.  Has 
it  been  improved  or  not?  In  what  respects?  The  topog- 
raphy is  quite  different.  Is  it  better  or  worse  for  man^s 
use  in  agriculture?  For  transportation?  The  multi- 
tude of  lakes  were  formed  by  the  glacier.  Are  they  an 
advantage  or  not?  How?  Most  of  the  waterfalls  are 
the  result  of  glaciation.  Are  they  a  benefit  or  not  ?  How  ? 
Enumerate  other  changes  caused  by  the  glacier,  stating 
whether  they  have  added  to  or  detracted  from  the  com- 
mercial value  of  the  region. 

BEFEBENCES 

Russell,  Glaciers  of  North  America,  Ginn  &  Co.,  1897. 
Shaler  and  Davis,  Glaciers,  Houghton,  Mifflin  &  Co.,  1881. 
Wright,  Ice  Age  in  North  America,  D.  Appleton  &  Co.,  1890. 
Salisbury,  Glacial  Geology,  Vol.  V.,  Geol.  Surv.  N.  X,  1902. 
Chamberlin,   The  Rock  Scorings  of  the  Great  Ice  Invasion, 

Tfrh  An.  Report  U.  S.  Geol.  Surv.,  p.  155. 
Upham,  Glacial  Lake  Agassiz,  Mon.  25,  U.  S.  Geol.  Surv. 
Geikie,  The  Great  Ice  Age,  D.  Appleton  &  Co.,  N.  Y.,  1895. 
Tarr,  Phys.  Geog.  of  N.  Y.  State,  Macmillan  Co.,  N.  Y.,  1902. 


168  PHYSICAL  GEOGRAPHY 

Davis,    The    Sculpture    of    Mountains    by    Glaciers,    Scottish 

Geog.  Mag.,  Feb.,  1906.,  p.   76. 
Fairchild,  Glacial  Lake  Iroquois,  N.  Y.  State  Museum,   20th 

An.  Report  State  Geol.,  1900,  Albany,  N.  Y. 
Fairchild,    Drumlins   of   New   York,    Bull.    Ill,   N.    Y.    State 

Museum. 


CHAPTER  V 

THE  OCEAN 

A  few  centuries  ago  the  ocean  was  an  impassable  bar- 
rier to  man ;  now  it  is  the  greatest  and  best  of  all  his  high- 
ways. Commercial  products  can  be  transported  much 
cheaper  across  the  ocean  than  the  same  distance  across  the 
continent,  to  say  nothing  of  the  greater  ease  and  comfort 
to  the  traveller.  Two  of  the  principal  factors  in  bringing 
about  this  change  are  the  use  of  the  mariner's  compass 
and  the  improvement  in  steam  navigation. 

134.  Size  of  the  Ocean.— The  ocean  is  the  irregular 
body  of  salt  water  surrounding  and  separating  the  con- 
tinents and  containing,  it  is  estimated,  about  1,300  quad- 
rillion tons  of  water.  It  covers  about  72  per  cent  of  the 
earth's  surface,  or  143,259,000  square  miles,  of  which 
7  per  cent  or  10,000,000  square  miles  lies  on  the  con- 
tinental shelf. 

135.  The  continental  shelf  is  the  shallow  margin 
of  the  ocean  bordering  the  continents.  It  varies  in  width 
from  a  fraction  of  a  mile  to  more  than  100  miles.  From 
the  -outer  or  ocean  margin  of  the  shelf  there  is  a  steep 
slope  or  descent  down  to  the  ocean  depths  forming  the 
sides  of  the  basin.  In  other  words,  the  ocean  basins  are 
full  to  overflowing  and  the  overflow  extends  out  over  the 
border  of  the  land  areas  forming  an  irregular  belt  of 
shallow  water,  the  continental  shelf,  which  corresponds 
in  a  way  to  an  irregular  rim  of  the  submerged  basin. 
Owing  to  elevations  and  depressions  of  the  earth's  crust 
the  width  of  this  shallow  water  zone  varies  greatly  from 

169 


170 


PHYSICAL  GEOGRAPHY 


time  to  time.  A  depression  of  the  continent  causes  a 
further  advance  of  the  water  on  the  land  and  the  con- 
tinental shelf  is  wider.  An  elevation  of  the  land  area 
causes  the  recession  of  the  shore  line,  the  emergence  of  the 
shallow  sea  bottom  and  the  continental  shelf  is  narrower 


j-o^T^Conlinent 

o 
,^  CO      Continental  shelf 

^¥m%/. 

/""       /  /  ? 

V        Ocean    Basin 

y?7/777;fii^w7/7, 

Fig.  125.  Vertical  section  across  a  continental  shelf  showing  its  relation  to 
the  continent  and  ocean  basin.  It  varies  from  one  or  two  to  100  miles 
or   more   in   width. 


and  the  continent  larger.  Considerable  portions  of  all  of 
the  continents  have  in  ages  past  been  covered  by  the  sea 
and  formed  part  of  the  continental  shelf  during  their  sub- 
mergence.   (See  fig.  125.) 

136.  Mediterraneans.— Besides  the  open  ocean  there  are 
several  smaller  divisions  partially  separated  from  it,  the  largest 
of  which  is  the  Mediterranean  Sea,  whose  depth  is  nearly  as 
great  as  that  of  the  great  oceans.  It  is  almost  entirely  sepa- 
rated from  the  open  sea,  being  connected  with  the  Atlantic  by 
the  narrow  strait  of  Gibraltar  and  with  the  Indian  ocean  by  the 
Suez  canal  and  the  Red  Sea. 

Other  mediterranean  seas  are  the  Gulf  of  Mexico,  the  Car- 
ribbean,  China,  and  Japan  seas,  the  surface  portions  of  which 
are  not  as  nearly  surrounded  by  land  as  the  Mediterranean  Sea, 
but  their  deep  basins  are  surrounded  by  land. 

137.  Composition  of  Sea  Water.— Sea  water  contains 
much  mineral  matter  in  solution,  the  average  being  about 
3%  per  cent  but  it  varies  considerably  in  different  parts 
of  the  ocean.  The  inflow  of  a  great  river  like  the  Amazon 
or  the  Mississippi,  or  excessive  evaporation  in  certain  local- 
ities produces  local  variations  in  the  percentage  of  salt. 
About  three-fourths  of  the  mineral  matter  held  in  solution 


THE  OCEAN  171 

is  sodium  chloride  or  common  salt.  The  remainder  consists 
largely  of  magnesium,  calcium,  and  potassium  salts.  There 
are  minute  quantities  of  other  elements. 

Chemical  composition  of  the  salts  of  average  sea-  water : 

Sodium  chloride  77.758% 

Magnesium    chloride    10.878 

Magnesium    sulfate    4.737 

Calcium  sulfate    3.600 

Potassium    sulfate     2,465 

Calcium   carbonate    345 

Magnesium   bromide    217 

100.000 
Besides  the  solid  salts  dissolved  in  the  sea  water,  there  is  a 
large  quantity  of  the  gases  of  the  atmosphere,  which,  like  the 
salts,  vary  greatly  in  quantity  in  different  parts  of  the  oceaoi 
and  in  the  same  part  at  different  times.  Fishes  and  other  animals 
of  the  sea  obtain  the  oxygen  necessary  for  life  from  the  sea  water. 
The  carbonate  of  lime  in  the  limestone  beds  on  the  land  is 
dissolved  by  carbonic  acid  in  the  groundwater  and  carried  into 
the  sea  in  solution.  When  the  corals  and  other  animals  se- 
crete the  lime  carbonate  in  their  skeletons  or  shells,  the  car- 
bonic acid  that  was  holding  it  in  solution  is  set  free  and  part  of 
it  at  least  goes  back  into  the  atmosphere. 

138.  Circulation  of  Salts  of  the  Ocean.— The  water 
flowing  into  the  sea  carries  salts  in  solution,  while  that 
which  is  evaporated  is  nearly  pure  water,  which  would 
apparently  cause  an  accumulation  of  salt  in  the  ocean. 
On  the  other  hand,  it  is  probable  that  much  of  the  salts 
carried  to  the  ocean  are  those  that  were  formerly  taken 
from  the  ocean.  The  great  beds  of  rock  salt  in  central 
New  York  and  elsewhere  were  formerly  in  the  sea.  Nearly 
all  the  great  beds  of  limestone  over  all  the  continents  were 
deposited  in  the  sea  from  materials  taken  from  the  sea 
water  and  are  now  being  returned  to  the  sea  to  be  again 
extracted  from  the  solution  by  animals  and  plants  to  form 
new  beds  of  limestone  over  the  sea  bottom.     Therefore,  the 


172 


PHYSICAL  GEOGRAPHY 


salts,  like  the  water,  circulate  from  the  ocean  to  the  conti- 
nents and  back,  and  it  is  not  possible  from  our  present 
knowledge  to  say  whether  or  not  the  sea  water  is  becoming 
denser. 

Density  of  sea  water  varies  with  the  temperature,  the 
composition  and  the  pressure.  There  is  an  increase  in  dens- 
ity with  decrease  in  temperature  to  near  the  freezing  point. 
It  expands  and  becomes  lighter  as  it  freezes.  The  average 
density  of  the  surface  of  the  sea  water  at  60  degrees  F.  is 
about  106.     There  is  a  slight   increase  in  density  to  the 

bottom  due  to  the  pres- 
sure of  the  overlying 
water.  An  increased  per- 
centage of  salts  in  solu- 
tion causes  a  correspond- 
ing increase  in  density. 

139.  Sounding  and 
Dredging.— Much  defi- 
nite knowledge  concern- 
ing the  sea  bottom  and 
the  deep  portions  of  the 
sea  has  been  obtained  in 
the  last  half  century  by 
improved  methods  of 
sounding  and  dredging. 
The  previous  explorations 
had  been  in  delineating 
the  shore  lines  of  the 
continents  and  islands. 
With  the  adoption  of 
modern   sounding   lines 


w 


"zr 


Fig.       126.      Sounding      apparatus.         The  j     j       j                                xi    u 

large    ball,    B,    weighs    several    hundred  and    drcdgCS,    a   nCW    Iield 

pounds     and     is     mechanically     detached  q£      investigation       WaS 
from     the     water     bottle,     W,     when     it 

strikes  the  bottom  of  the  ocean.  Opened,  namely,  the  ocean 


THE  OCEAN 


173 


bottom,  the  study  of  which  has  given  rise  to  the  new  science, 
Oceanography. 

Soundings  are  made  with  fine  steel  wire  (why  not 
rope?)  to  which  a  sinker  in  the  form  of  a  heavy  iron  ball 
like  a  cannon  ball  is  attached  in  such  a  way  that  it  is  re- 
leased when  it 
strikes  the  bottom. 
Why  are  the  sinkers 
left  on  the  bottom 
of  the  ocean?  Sam- 
ples of  water  are 
obtained  from  dif- 
ferent depths  by  at- 
taching to  the  wire 
at  definite  intervals 
brass  tubes,  called 
water  bottles,  so 
constructed  that 
they  remain  open  in 
descent  but  are  au- 
tomatically closed 
as  soon  as  lifting 
begins. 

The  temperature 
of  the  ocean  deeps 
is  obtained  by  self- 
recording  thermome- 
ters. These  like  the 
water  bottles  may  be 
attached  to  the  line 
at  different  places, 
so  that  a  single 
sounding  may  give, 
besides    the    depth, 


Fig.  127.  Two  types  of  dredges  used  in  collect- 
ing specimens  of  the  mud  and  life  forms 
from  the  bottom  of  the  ocean.  The  bag 
is  attached  to  a  long  wire  from  the  ship, 
and  is  dragged  along  the  bottom  scooping  up 
material.  Some  low  forms  of  life  are  caught 
in  the  tangles  below  the  dredge  and  brought 
to  the  surface  in  that  way.  1,  Chester  rake 
dredge.     2,  Blake  dredge.      (U.  S.  Fish  Com.) 


174  PHYSICAL  GEOGRAPHY 

samples  of  the  water  and  the  temperature  at  several  differ- 
ent places  between  the  surface  and  the  bottom.  Moreover, 
a  sample  of  the  bottom  mud  may  at  the  same  time  be  ob- 
tained by  collecting  that  which  sticks  to  the  water  bottle 
at  the  end  of  the  wire. 

Specimens  of  the  bottom  sediment  are  generally  ob- 
tained along  with  specimens  of  the  life  in  trawls  or 
dredges,  consisting  of  strong  nets  having  an  iron  rim  and 
laden  with  weights.  These  nets  when  dragged  along  the 
sea  bottom  scoop  up  masses  of  the  soft  mud,  ooze,  and 
specimens  of  such  forms  of  life  as  there  exist.     (Fig.  127.) 

There  is  another  method  of  sounding  by  means  of  an  instru- 
ment which  records  the  pressure.  One  advantage  of  this 
method  lies  in  the  fact  that  it  can  be  used  without  stopping  the 
vessel,  as  it  is  independent  of  the  length  of  line. 

140.  The  Deeps.— Scattered  over  the  floor  of  the 
ocean  basins  are  deep  depressions— the  so-called  deeps  or 
anti-plateaus y  which  extend  below  the  ocean  bottom  to 
about  the  same  extent  that  the  plateaus  rise  above  the 
general  level  of  the  continents. 

The  deepest  known  point  in  the  ocean  is  the  Challenger 
deep,  31,600  feet,  near  our  insular  possession  Guam.  The 
Aldrich  deep  near  New  Zealand  is  30,930;  the  deepest 
sounding  in  the  Atlantic  is  near  Porto  Rico,  27,930.  The 
Atlantic  ocean  is  generally  deeper  near  the  sides,  (15,000 
feet  to  18,000  feet)  than  in  the  middle,  (9,000  to  12,000 
feet).  The  elevated  area  of  the  mid-ocean  bottom  is 
called  the  Telegraph  plateau  and  across  it  extends  the 
several  Atlantic  cables  from  North  America  to  Europe. 

The  average  depth  of  all  the  oceans  is  about  12,000  to 
15,000  feet,  which  is  nearly  six  times  the  average  height 
of  the  lands  above  the  ocean  level.  It  is  estimated  that  if 
all  the  continents  and  islands  were  thrown  in  the  sea  the 
average  depth  would  be  nearly  two  and  a  half  miles. 


THE  OCEAN  175 

141.  Temperatures  of  the  Ocean.— The  surface  of  the 
ocean  is  heated  by  the  sun's  rays,  but  these  probably  do 
not  produce  any  perceptible  effect  below  a  few  hundred 
feet.  Since  water,  like  air,  grows  lighter  as  it  is  heated, 
the  surface-heated  waters  do  not  sink  and  hence  do  not 
reach  the  ocean  bottom  or  any  great  depth  in  the  ocean. 
The  surface  water  in  the  equatorial  region  is  heated  to 
about  80  degrees  F.  At  the  poles  it  is  frozen  part  of  the 
year  and  near  the  freezing  point  most  of  the  time.  Since 
the  colder  water  becomes  heavier  and  sinks,  all  the  water 
of  the  polar  oceans  is  near  28  degrees  F.,  the  freezing 
point  of  salt  water.  As  there  is  a  slow  creep  of  this  water 
along  the  ocean  bottom  towards  the  equator,  the  deeper 
portions  of  the  ocean,  even  in  the  equatorial  regions,  are 
very  cold.  Specimens  of  ooze  and  mud  brought  from  the 
ocean  bottom  in  the  tropics  show  temperatures  at  or  near 
the  freezing  point. 

The  body  of  the  ocean  water  has  a  rather  uniform  tem- 
perature. Even  the  surface  waters  change  but  little  in 
comparison  with  the  land  temperatures,  the  daily  change 
of  the  surface  rarely  exceeding  two  or  three  degrees  and 
the  yearly  maximum  range  being  fifteen  degrees. 

Soundings  of  the  Challenger  in  the  Atlantic  ocean,  3 J/2 
degrees  south  of  the  equator,  show  the  following  temper- 
atures : 

Surface  78   degrees   Fahr. 

270  feet  deep  68 

960     "        "  '  50 

1920     "        "  41 

9000     "         "  36.5 

15200     "        "  33 

The  temperature  of  inland  seas  or  mediterraneans  is  higher 
than  that  of  the  bordering  ocean  at  corresponding  depths  be- 
cause they  have  no  connection  with  the  polar  waters  and  do 
not  have  a  temperature   at  the  bottom  lower   than  that  at  the 


176 


PHYSICAL  GEOGRAPHY 


lowest  place  in  the  strait  connecting  them  with  the  open  sea  or 
the  coldest  water  formed  in  the  winter  season  of  the  area.  (See 
fig.  128.) 


ft 


CARIBBEAN    SEA. 

70-80° 


AtL ANTIC   OCEAN 

s^  70-80° 


-39  H-- 
39M° 


Fig.  128.  Diagram  showing  relation  of  temperatures  in  a  mediter- 
ranean sea  to  corresponding  depths  in  the  open  sea.  The  cold 
waters  of  the  deep  sea  do  not  rise  and  hence  do  not  pass  the 
shallow  water  of  the  connecting  strait. 

142.  Waves. — Waves  are  formed  on  the  sea  or  any 
body  of  water  by  the  friction  of  the  wind  blowing  across 
it,  causing  the  surface  water  to  move  up  and  down,  back 
and  forth,  each  particle  of  water  traversing  an  elliptical 
path.  Generally  the  backward  movement  equals  the  for- 
ward and  the  water  comes  to  rest  where  it  started,  except 
where  the  waves  curl  and  break,  when  the  top  of  the  wave 
is  driven  forward.  Where  the  wind  continues  for  some 
time  in  the  same  direction,  considerable  quantities  of 
water  are  driven  forward  and  heaped  up  on  the  windward 
shores.     (Fig.  129.) 


Fig.  129.  Diagram  illustrating  the  orbital  movement  of  water  particles  in 
waves.  The  water  rises  in  front  of  the  advancing  wave  and  sinks  after 
the  passing  of  the  crest,  each  particle  traversing  a  circular  or  elliptical 
path.  A  B  level  of  water  at  rest.  C  C  length  of  wave  from  crest  to 
crest.      D    D'   height   of  wave. 


THE  OCEAN 


177 


Size  of  waves.  The  stronger  the  wind  the  larger  the  waves 
that  are  formed.  The  height  of  the  wave  measured  from  the 
bottom  of  the  trough  to  the  top  of  the  crest,  is  sometimes 
thirty  feet  or  more,  rarely  reaching  a  height  of  fifty  feet  in  the 
open  sea.  The  length  of  the  wave  varies  from  a  few  feet  to 
1500  feet  or  more,  much  more  in  the  earthquake  waves,  and  the 
velocity  varies  from  20  to  60  miles  an  hour.  The  visible  side  of 
the  advancing  wave  is  the  front,  the  opposite  side  the  back  of 
the  wave. 

The  size  of  the  wave  increases  with  the  density,  area,  and 
depth  of  the  water;  hence  the  ocean  waves  are  larger  than 
those  on  lakes  or  rivers. 


Pit),  lao.     Breakers  and  surf  on  'boulder  beach.      (M.  S.  Lovell.) 


143.  Breakers.— As  the  waves  of  the  open  sea  ap- 
proach the  shore,  where  there  is  not  sufficient  depth  of 
water  to  form  the  front  of  the  advancing  wave  the  top 
moves  forward,  breaks  off,  and  falls  as  foam  to  be  caught 
by  the  advancing  wave  and  carried  forward  until  it  breaks 
again,  in  this  way  forming  the  so-called  ''breakers"  along 
the  shore.     It  is  these  breakers  that  are  so  destructive  to 

12 


178  PHYSICAL  GEOGRAPHY 

boats  and  other  property.  Hence,  when  vessels  cruising 
along  the  shore  find  a  storm  coming,  if  there  is  no  good 
harbor  near  at  hand,  they  sail  for  the  open  sea  to  avoid  the 
destructive  breakers  of  the  shallow  water.  The  white  foam- 
ing waters  produced  by  the  breakers  on  the  shore  are  called 
the  surf.  The  turbulent  waters,  when  not  too  violent,  are  at- 
tractive to  the  surf  bathers. 

144.  Undertow.— The  undertow  is  a  backward  move- 
ment along  the  bottom  from  the  shore  towards  the  open 
sea.  The  water  that  is  carried  forward  and  heaped  up  on 
the  shore  by  the  breakers  and  surf  cannot  return  sea-ward 
on  the  surface  because  of  the  incoming  waves,  so  it  flows 
back  along  the  bottom,  forming  the  undertow  which  so 
often  proves  dangerous  to  the  surf  bathers,  who  are 
caught  by  it  and  carried  out  into  deep  water  and  drowned. 
The  fine  material  that  is  ground  up  by  the  waves  on  the 
beach  is  carried  back  into  the  deeper  water  by  the  under- 
tow and  spread  out  in  beds  of  gravel,  sand,  and  clay  which 
may  later  be  elevated  and  form  part  of  the  stratified  rocks 
of  the  continent. 

145.  Earthquake  Waves.— When  an  earthquake  shock  takes 
place  beneath  the  bed  of  the  sea,  it  sometimes  causes  the  ele- 
vation of  the  surface  of  the  water  over  a  large  area,  which 
spreads  out  in  long,  low  waves,  having  great  velocity.  As  these 
waves  approach  the  shore,  they  decrease  in  velocity  but  increase 
in  height,  piling  up  the  water  on  the  shore  with  great  force, 
causing  at  times  enormous  destruction  of  life  and  property. 

During  the  disastrous  earthquake  that  destroyed  Lisbon  in 
1755,  the  first  shock  caused  the  people  who  were  not  killed  to 
leave  their  houses.  Most  of  them  assembled  on  the  new  marble 
quay,  when  the  sea  wave,  50  feet  or  more  in  height,  swept  in 
with  great  force,  destroying  nearly  60,000  people. 

The  great  volcanic  eruption  and  accompanying  earthquake 
shock  at  Krakatoa  in  1883  produced  sea  waves  that  spread 
around  the  world.  On  the  coasts  near  the  eruption,  waves  70 
feet  or  more  in  height  rushed  on  the  shore,  destroying  many  vil- 


THE  OCEAN 


179 


lages  and  thousands  of  people.  So  powerful  were  the  waves 
that  a  large  ocean  vessel  was  swept  a  mile  and  a  half  inland 
and  left  there  by  the  retreating  wave.  Earthquake  waves  are 
sometimes  wrongly  called  tidal  waves. 

146.  Effects  of  the  Waves.— (1)  One  of  the  most  con- 
spicuous effects  of  the  waves  is  the  modification  of  the 
shore  line  produced  by  their  erosive  action.     In  this  work 


Fig.  131.  Wave  eroded  shore,  Maryland.  The  indentations  are  worn  by  the 
waves  assisted  by  gullies.  The  material  is  diatomaceous  earth.  (Maryland 
Geological  Survey.) 


the  common  wind  and  storm  waves  are  assisted  by  the 
tidal  and  the  earthquake  waves.  They  wear  away  rocks 
in  some  places  and  build  up  bars  and  reefs  in  others.  The 
softer  rocks  are  worn  away  first,  forming  bays  and  inlets 
between  the  harder  rocks  which  form  the  headlands,  or  in 
some  cases  islands.     (See  Ciiapter  VI). 

(2)  The  waves  aerate  the  waters  of  the  ocean  by  stir- 
ring them  up  and  thus  exposing  larger  surfaces  to  the 
action  of  the  atmosphere;  also  by  blowing  over  the  crests 
of  the  waves,  thus  inclosing  the  air  in  the  waters.     This 


180 


PHYSICAL  GEOGRAPHY 


action  serves  to  oxidize  the  decaying:  organic  matter  and 
thus  purify  the  waters;  it  also  furnishes  oxygen  for  the 
animals  living  in  the  sea. 

(3)   The. waves   exercise   enormous  mechanical   power, 
part  of  which  is  utilized  by  man  to  ring  the  bell  and  blow 


Fig.  132,     Low  tide  in  Bay  of  Fundy,  near  Gaspareaux  River.     See  Fig. 
133.      (Roland  Hayward,  1903.) 

the  whistle   on   the   harbor  buoys.     This  power   is  some- 
times used  to  pump  water,  or  open  flood  gates. 

(4)  The  waves  are  frequently  destructive  to  life  and 
property.  During  violent  storms  they  destroy  sea  walls, 
docks,  lighthouses  and  other  property  on  shore,  and  fre- 
quently overwhelm  and  destroy  boats.  The  destructive 
effect  of  the  waves  on  boats  in  the  open  sea  is  materially 
lessened  and  often  the  vessel  is  saved  by  spreading  a  little 
oil  on  the  water.  The  disastrous  effects  of  the  waves  are 
produced  by  the  breaking  of  the  wave,  when  the  top  curls 


THE  OCEAN  181 

over  and  falls  upon  the  boat.  A  little  oil  on  the  water 
spreads  rapidly  even  in  the  face  of  the  wind,  and  decreasejj 
the  friction  enough  to  permit  the  crest  of  the  wave  to 
settle   back   quietly   without   breaking.     Small   boats  can 


Fig.    133.     High   tide   in   Bay   of  Fundy,    same   point   as   FiG.    132.      (Roland 
Hayward,  1903.) 

safely  ride  the  largest  waves  as  long  as  the  waves  do  not 
break  and  fall  into  the  boat. 

147.  Tides  and  Tidal  Waves.— At  all  points  on  the 
shore  of  the  ocean  the  water  rises  and  falls  twice  each  da^^ 
It  rises  steadily  for  about  six  hours  until  it  reaches  its 
highest  level,  high  tide,  and  then  subsides  for  about  six 
hours,  until  it  reaches  its  lowest  level,  low  tide,  when  it 
again  rises.  The  period  of  rising  is  not  always  uniform 
with  the  period  of  falling,  but  the  average  of  the  sum  of 
the  two  is  equal  to  12  hours  and  26  minutes.  Figs.  132  and 
133.) 


182  PHYSICAL  GEOGRAPHY 

Twice  each  month  the  tides  reach  a  maximum  height, 
the  spring  tide,  and  twice  they  reach  the  minimum  height, 
the  neap  tide.  The  incoming  tide  is  called  the  flood  tide, 
the  outgoing  the  ebb  tide.  Slack  water  is  the  interval  be- 
tween the  two. 

In  shallow  harbors  the  hour  of  departure  of  ocean  steamers 
is  usually  determined  by  the  time  of  high  tide,  as  they  can  then 
float  with  safety  over  the  bars  and  shallow  places  which  they 
could  not  pass  at  low  tide.  Finding  the  time  when  high  tide  will 
occur  at  any  place  is  called  "establishing  the  port." 

On  the  open  sea  the  rise  and  fall  of  the  tide  is  not  percep- 
tible, so  low  and  broad  is  the  wave.  In  bays  and  estuaries  where 
the  tidal  wave  is  confined  and  restricted,  it  frequently  rises  to 
great  heights.    In  the  Bay  of  Fundy  on  the  coast  of  Nova  Scotia 


Fig.  134.     Tidal  flats. — Low  tide  in  Basin  of  Mines,  N.   S.     The  area  is 
covered  with  water  at  time  of  high  tide.      (S.  R.  Stoddard.) 

the  tide  rises  to  a  height  of  50  feet  or  more.  Similar  high  tides 
occur  in  the  Bristol  channel.  In  both  places  the  tide  is  not  con- 
spicuously high  at  the  comparatively  wide  mouth  of  the  bay,  but 
as  the  low,  long  wave  advances  up  the  ever-narrowing  channel 
the  waters  begin  to  pile  up  until  they  reach  a  maximum  at  or 
near  the  head  of  the  bay.     (Figs.  132  and  133). 

148.     Tidal  Wave  in  Rivers.— In  certain  places  the 


THE  OCEAN  183 

tidal  wave  meets  opposition  in  the  current  of  a  river  and 
at  times  the  waters  rise  into  a  high  wave  commonly  known 
as  the  hore  or  eagre  which  rushes  up  the  river,  often  with 
high  velocity,  causing  great  destruction  along  the  banks 
and  at  times  to  shipping  in  the  river.  On  the  Amazon 
.River  this  tidal  wave,  known  as  the  pororoca,  extends  for 
several  hundred  miles  up  the  river  with  great  destruction 
to  the  bordering  forests.  Similar  waves  often  prove  very 
destructive  to  shipping  on  the  Hoang  Ho  (River)  in  China 
and  the  Seine  River  in  France. 

Tidal  race.  In  Long  Island  Sound  a  low  tide  from  the  east 
meets  a  high  tide  from  the  west  at  Hell  Gate  and  six  hours  later 
the  conditions  are  reversed.  At  both  times  the  water  rushing 
through  the  narrow  channel  with  great  velocity  proved  very 
destructive  to  shipping  until  the  channel  was  widened  by  blast- 
ing away  the  rocks.     Such  a  current  is  called  a  tidal  race. 

A  somewhat  similar  but  more  complex  meeting  of  the  tides 
in  the  North  Sea  forms  the  dreaded  Maelstrom  oft  the  coast  of 
Norway. 

149.  Cause  of  the  Tides.— The  close  relation  existing 
between  the  time  of  the  tides  and  the  time  of  the  succes- 
sive crossings  of  the  meridian  by  the  moon  was  known 
for  a  long  time  before  it  was  suggested  that  gravitative 
attraction  was  probably  the  cause  of  the  tides.  The  at- 
traction of  the  moon  causes  the  heaping  up  of  the  waters 
on  the  side  towards  the  moon,  because  they  are  nearer  and 
because  the  waters  respond  more  readily  to  the  gravitative 
pull  than  do  the  more  rigid  rocks.  For  the  same  reason 
the  waters  would  rise  on  the  side  opposite  the  moon.  The 
heaping  up  of  the  waters  on  opposite  sides  of  the  globe 
causes  a  lowering  of  the  waters  or  low  tide  at  the  inter- 
mediate points.     (See  fig.  135.) 

The  sun  also  causes  a  tide,  but  so  much  smaller  than 
that  caused  by  the  moon  that  it  is  rarely  noted  except 


184 


PHYSICAL  GEOGRAPHY 


when  it  coincides  with  that  of  the  moon  or  is  directly  op- 
posed to  it. 

Spring  and  neap  tides.     The  sun  tide  coincides  with  the  moon 
tide  when  the  sun  and  moon  are  in  line  with  the  earth,  either 


o 


0 — 


6 


a. 


B 


Fig.  135.  Diagram  showing  position  of  the  sun  and  moon  during  spring  and 
neap  tides.  A  spring  tide,  the  result  is  the  same  at  full  moon  when  the 
moon  is  on  the  oposite  side  from  the  sun.  B,  neap  tide.  First  quarter; 
the   result   is  the  same   at  the   third   quarter  of  the   moon, 

on  the  same  side,  as  at  new  moon,  or  on  opposite  sides,  as  at 
full  moon.  The  tides  are  then  equal  to  the  sum  of  the  two,  the 
greatest  for  the  month,  and  are  called  spring  tides. 

During  the  moon's  quarter  the  sun  and  moon  are  at  right 
angles  to  each  other  from  any  point  on  the  earth,  when  the  tide 
equals  the  difference  between  the  two  and  hence  is  the  lowest 
for  the  month,  the  neap  tides,  which  come  at  the  first  and  the 
third  quarters. 

Owing  to  the  inertia  of  the  water,  it  takes  some  time  for  the 
full  effect  of  the  moon's  attraction  to  manifest  itself,  so  that 
high  tide  Is  not  directly  underneath  the  moon,  but  some  distance, 
at  times  some  hours  behind  it.  This  is  the  lag  of  the  tides. 
The  regularity  of  the  movement  of  the  wave  is  disturbed  very 
much  by  the  continental  and  insular  land  masses,  so  that  in. 
many  places  the  tidal  movements  become  quite  comple«. 


THE  OCEAN  185 

150.  Ocean  Currents.— The  very  slow,  almost  imper- 
ceptible movements  of  the  ocean  waters  are  called  creep; 
the  faster  but  still  very  slow  movements  are  called  drift; 
the  faster,  more  conspicuous  movements  are  called  cur- 
rents, and  the  more  rapid  currents  are  called  streams.  In 
^the  waves  the  movement  of  the  water  is  mainly  up  and 
down,  but  in  the  strong  wind  waves  where  they  break  at 
the  top,  forming  the  "white  caps,"  there  is  a  forward 
motion  at  the  top  of  the  wave  which  is  blown  over  and 
driven  forward  by  the  wind.  A  continuation  of  this 
movement,  as  in  the  belt  of  constant  winds,  would  pro- 
duce a  surface  current. 

151.  Causes  of  Ocean  Currents.— The  causes  given  for 
ocean  currents  are  winds,  differences  in  temperature  and 
pressure,  and  the  rotation  of  the  earth.  There  is  some 
difference  of  opinion  concerning  the  relative  importance 
of  these.  The  movement  of  the  winds  is  probably  the  most 
important  cause  of  all.  The  difference  in  temperature  be- 
tween the  warm  tropical  waters  and  the  cold  polar  waters 
would  cause  convectional  movements.  The  ultimate  cause 
in  both  cases  is  difference  in  temperature,  but  in  the  first 
case  it  produces  movements  of  the  atmosphere,  which  in 
turn  cause  movements  of  the  water.  The  rotation  of  the 
earth  probably  produces  movement  of  the  ocean  waters  to 
some  extent.  The  direction  of  the  movements  is  influenced 
by  the  rotation  of  the  earth  and  by  the  outline  of  the  con- 
tinents. 

152.  Currents  in  the  Atlantic— In  the  Atlantic  Ocean 
there  is  a  current  westward,  in  the  equatorial  regions,  to 
the  South  American  coast,  where  part  of  it  is  deflected 
into  the  South  Atlantic  and  part  north.  The  north 
branch  divides  at  the  West  Indies,  '  part  of  it  passing 
through  the  Carribbean  Sea  and  the  Gulf  of  Mexico, 
whence  it  emerges  as  the  Gulf  Stream.     After  joining  the 


186 


PHYSICAL  GEOGRAPHY 


other  portion  of  the  equatorial  current  east  of  Florida,  it 
continues  northeast  across  the  ocean  as  the  Atlantic  drift, 
dividing  again  west  of  Europe  where  a  portion  of  it  con- 
tinues northeast  until  it  is  lost  in  the  Arctic  ocean.  The 
other  part  of  the  Atlantic  drift  turns  southward  along 
the  coast  of  Spain  and  Portugal  and  northwest  Africa, 


^AUs^ALiA^  ^       ^    ^     ^SOUTH     PACIFIcy^    M  "  Fl/^' A  T  L  .»  N  T  i  c  '     'iK-   yy     C '    o   o   V 


\- r""^.-*'l.N--''T,,--'A.';T.R'''c  ,-T'    l-'-fc'  E    ..D-D-  Y" 


Cbait  «t  U>e  Ocean  Cturents. 

Fig.   136.     Map   of  ocean   currents.     The  solid  lines   are   warm  currents;   the 
dotted  lines  are  cold  currents.     Sargasso  seas  in  each  of  the  eddies. 


until  it  finally  joins  the  equatorial  current  to  be  again 
turned  westward  across  the  Atlantic,  thus  completing  the 
circuit  of  the  North  Atlantic  Ocean.  Trace  out  in  a 
similar  way  from  the  map  the  warm  currents  and  then 
the  cold  ones  of  the  other  oceans.     (Fig.  136.) 

153.  Sargasso  Seas.— The  central  portion  of  the  North  At- 
lantic, the  part  surrounded  by  the  current  just  described,  hag 
no  continued  movement  but  corresponds  to  the  central  portion 
of  a  great  eddy.  It  is  called  the  Sargasso  sea  from  the  abun- 
dance of  the  seaweed  Sargassum  which,  because  of  the  numerous 
air  sacs  along  the  stem,  floats  on  the  surface  of  the  ocean. 
Under  the  combined  action  of  the  eddying  waters  and  the  shift- 
ing winds,  this  floating  weed  accumulates  in  places  in  such  dense 


THE  OCEAN  187 

masses  as  to  seriously  retard  the  progress  of  ships.  It  was  part 
of  the  Sargasso  sea  that  Columbus  encountered  in  his  first  voy- 
age, where  his  sailors  became  frightened,  thinking  they  might 
never  escape.     Where  are  the  other  sargasso  seas? 

154.  Drift  of  Cold  Waters.— Slow  movements  of  the 
ocean  water  are  called  drift  or  creep.  There  is  an  ex- 
tensive creep  of  the  cold  polar  waters  toward  the  equato- 
rial regions  which  appears  as  surface  movements  only  in 
high  latitudes,  and  only  locally  does  the  movement  form 
true  currents.  When  the  south-moving  cold  polar  waters 
meet  the  north-moving  warm  currents  or  drift,  they  sink 
underneath  the  warm  waters  and  continue  to  creep 
equatorward  along  the  ocean  bottom.  Ferrel's  law  (see 
index)  about  the  movements  of  the  atmosphere  applies 
equally  well  to  the  ocean  currents.  From  the  map  show 
or  explain  the  relation. 

155.  Effects  of  Ocean  Currents.— (1)  The  movement 
into  the  higher  latitudes  of  large  bodies  of  warm  water, 
like  the  Gulf  Stream  in  the  Atlantic  and  the  Japan  cur- 
rent in  the  Pacific,  carries  with  it  tropical  heat  which 
tempers  the  climate  in  a  marked  degree.  Likewise  the 
polar  currents  and  the  drift  bring  the  cold  of  the  polar 
regions    into    the    lower    latitudes    and    cool   the   climate. 

(2)  Ocean  currents  affect  navigation  by  hastening  or 
retarding  the  speed  of  vessels,  depending  upon  whether 
they  are  going  with  or  against  the  current.  Sometimes 
they  cause  vessels  to  drift  from  their  courses  on  to  dan- 
gerous coasts,  when  the  current  is  toward  the  shore  along 
which  the  vessel  is  sailing.  Because  of  these  shifting 
movements,  often  imperceptible,  the  navigator  must  use 
extra  precautions  in  planning  his  course  and  in  getting 
his  location  by  astronomical  observations.  Nansen  at- 
tempted to  reach  the  north  pole  by  getting  in  the  waters 
drifting  northward  and  permitting  his  vessel  to  be  frozen 


188  PHYSICAL  GEOGRAPHY 

in  the  ice  and  carried  along  with  the  drift.  The  lack  of 
definite  knowledge  concerning  the  polar  drift  resulted  in 
failure  to  reach  the  pole. 

(3)  Ocean  currents  distribute  plant  life.  The  islands 
of  the  sea  receive  seeds  which  have  drifted  from  distant 
lands.  Many  of  the  verdure-clad  islands  that  would 
otherwise  have  remained  barren  land  have  received  their 
vegetation  in  this  way.  In  a  similar  manner,  both  lower 
and  higher  forms  of  animal  life  have  been  carried  long 
distances  by  the  ocean  currents  and  thus  introduced  into 
other  lands. 

(4)  The  transference  of  water  from  one  part  of  the 
ocean  to  another  by  the  currents  prevents  stagnation  and 
makes  life  possible  by  carrying  oxygen  and  food  to  the 
different  organisms.  The  currents  are  instrumental  in 
causing  dense  fogs  by  bringing  cold  and  warm  water  and 
hence  cold  and  warm  air  together  in  large  quantities. 

Explain  the  cause  of  the  dense  fogs  that  occur  so  frequently 
on  the  fishing  grounds  of  the  banks  near  Newfoundland.  Since 
many  of  the  trans-Atlantic  steamers  pass  over  these  banks  the 
danger  of  collision  with  the  fishing  vessels  is  greatly  increased 
by  the  heavy  fogs.  Why  is  London  noted  for  its  fogs?  Why 
should  there  be  more  fog  at  San  Francisco  than  at  Denver? 

That  the  colder,  heavier  polar  waters  creep  along  the  ocean 
bottom  towards  the  equator  is  shown  by  the  low  temperature 
of  the  water  at  great  depths  in  tropical  regions.  This  move- 
ment is  probably  universal  but  very  slow  in  all  oceans.  The  in- 
creeping  cold  waters  replace  the  warm  water  carried  from  the 
warmer  regions  by  evaporation,  and  thus  complete  a  general 
circulation  of  all  the  oceanic  waters. 

THE    OCEAN    FLOOR 

156.  Topography  of  the  Ocean  Bottom.— The  broad- 
er general  features  of  the  ocean  bottom  are  not  greatly 
different  from  those  of  the  land  areas,  but  the  details  are 
decidedly    different.      There    are    plains,    plateaus,    and 


THE  OCEAN  189 

mountains,  but  there  is  an  almost  total  lack  of  valleys  and 
hills  that  mark  the  continental  land  areas.  Hence  if  the 
sea  bottom  were  exposed  to  view  one  would  be  impressed 
by  the  striking  monotony  of  the  scenery,  the  absence  of 
the  many  varied  forms  sculptured  on  the  land  by  rainfall, 
winds,  and  streams. 

In  places  on  the  continental  shelf  where  portions  of  the  land 
area  have  been  recently  submerged,  the  buried  hills  and  valleys 
are  not  entirely  obliterated. 

157.  Materials  on  the  Sea  Bottom.— The  materials 
on  the  sea  bottom  are  quite  varied  in  different  places,  but 
may  be  divided  into  those  on  the  continental  shelf  and 
those  over  the  deep  sea  basins.  The  first  would  include 
those  deposited  in  water  less  than  600  feet  deep  and  would 
consist  of  gravel,  sand,  and  mud;  coral,  and  other  organic 
deposits,  the  materials  of  which  are  derived  mainly  from 
the  lands. 

The  materials  eroded  from  the  land  by  the  rivers  and 
from  the  beach  by  the  ocean  waves  are  carried  out  and 
spread  over  the  sea  bottom  by  the  river  and  shore  currents, 
by  the  undertow  and  by  winds  which  carry  it  as  dust 
through  the  atmosphere.  The  sand  and  mud  are  carried 
in  suspension  and  dragged  along  the  bottom,  while  the 
lime  carbonate  for  the  limestones  is  carried  out  in  solu- 
tion. 

The  mechanical  sediments  are  generally  coarser  and 
thicker  within  a  few  miles  of  the  shore,  thinning  out  in 
the  deeper  waters.  The  calcareous  deposits  are  formed 
in  the  clearer  waters  which  are  comparatively  free  from 
sediments. 

158.  Deep  Sea  Deposits.— The  deeper  portions  of 
the  sea— the  deep  basins  outside  the  continental  shelf- 
are  covered  with  organic  oozes  and  fine  muds.  Some  of 
the  oozes  are  calcareous  or  limy,  and  some  silicious,  that 


190 


PHYSICAL  GEOGRAPHY 


is,  composed  of  silica.  The  most  common  of  the  calcar- 
eous oozes,  named  from  the  prevailing  forms  of  organic 
remains,  are  the  glohigerina  ooze  and  the  pteropod  ooze; 
the  siliceous  ones  are  the  radiolarian  ooze  and  the  diatom 
ooze.  The  first  three  are  minute  animal  forms;  diatoms 
are  microscopic  plants  of  varied  and  beautiful  forms 
which  live  in  both  salt  and  fresh  water.  (See  sec.  105, 
chapter  III.) 


Fig.  137.  Some  of  the  deep  sea  ooze  highly  magnified.  The  minute  animals 
live  at  or  near  the  surface  of  the  sea  biit  their  remains  sink  to  the  bottom 
where  it  forms  ooze.  A — foraminiferal  ooze  magnified  50  diameters 
from  depth  of  11,000  feet.  B — raliolarian  ooze  magnified  100  diameters 
from  depth  of  26,850    (one   of  the  deeps.) 

The  microscopic  plants  and  animals  which  form  the 
different  oozes,  live  on  or  near  the  surface  of  the  sea  and 
as  they  die,  their  remains  sink  to  the  bottom  and  accumu- 
late as  the  soft  ooze.  They  live  at  the  surface  in  shallow 
water  as  well  as  in  mid-ocean  but  there  is  so  much  other 
miaterial  on  the  bottom  of  the  shallow  seas  that  the  re- 
mains of  the  microscopic  organisms  are  generally  ob- 
scured, while  in  the  deep  sea  they  form  the  bulk  of  the 
material  on  the  bottom. 

In  many  places  around  the  border  of  the  oceanic  basins 
there  are  extensive  areas  of  fine  muds,  named  from  their 


THE  OCEAN  191 

color  blue,  red,  and  green.  In  the  deepest  portions  of 
the  sea  basins,  known  as  the  deeps,  the  bottom  is  covered 
with  a  red  clay,  the  origin  of  which  is  uncertain,  but  it  is 
probably  formed  in  part  at  least  of  volcanic  and  meteoric 
dust.  Glauconite  or  green  sand  covers  the  sea  bottom 
in  some  places. 

The  greater  part  of  the  continental  areas  is  covered  with 
rocks  that  were  formed  in  the  sea.  Even  the  greatest  mountain 
ranges  and  the  extensive  plateau  areas  are  largely  composed  of 
the  materials  of  former  sea  bottoms.  It  is  quite  probable  that 
portions  of  the  present  sea  bottom  over  the  continental  shelf  may 
in  the  future  become  land  areas  not  greatly  different  from  the 
present  lands. 

Most  of  the  rock  over  the  continents  consists  of  sand- 
stones, shales  and  limestones,  the  off-shore  shallow  water 
deposits;  yet  there  are  representatives  of  the  oozes  in  the 
chalk  beds  of  England,  Prance,  and  portions  of  the  United 
States.  There  are  diatomaceous  deposits  of  great  extent 
in  California,  and  smaller  deposits  at  Richmond,  Virginia, 
and  elsewhere. 

159.  Stability  of  Sea  Basins  and  Continents.— Despite  'the 
fact  that  the  continents  are  for  the  most  part  covered  with  sea 
bottom  deposits,  there  is  good  reason  for  thinking  that  there  is 
little  or  no  change  from  sea  basin  to  land  area  and  vice  versa. 
The  interchange  has  been  between  the  continents  and  the  con- 
tinental shelf.  At  the  present  time  the  area  of  the  continental 
shelf  is  about  10,000,000  square  miles.  From  time  to  time  por- 
tions of  it  are  elevated  above  the  sea  level  and  added  to  the  con- 
tinents and  portions  of  the  continents  are  depressed  and  added 
to  the  sea  area.  Diastrophic  movements  (compare  first  part  of 
Chapter  VIII)  of  this  kind  have  produced  many  changes  in  the 
land  and  sea  areas  during  the  past  geological  ages.  Through  them 
all  the  continents  have  probably  been  growing  larger  and  the 
continental  shelf  smaller.  The  ocean  basins  have  doubtless 
been  growing  deeper  but  not  larger  in  area. 

160.  Life  in  the  Ocean.— The  life  in  the  ocean  is  quite 
varied  and  in  places  very  prolific.     The  varying  physical 


192  PHYSICAL  GEOGEAPHY 

conditions  produce  three  rather  distinct  life  regions:  (1) 
the  continental  shelf  or  shallow  water  area,  (2)  the  bot- 
tom of  the  deep  sea  basins,  (3)  the  pelagic  life  or  that  on 
the  surface  of  the  open  sea. 

Life  on  the  shore  and  in  the  shallow  seas.     The  life  of 


Pig.  138.  Two  of  the  larger  animals  of  the  open  ocean.  The  upper  one  is  a 
grampus  or  whale  which  has  been  stranded  on  the  beach.  The  other 
is  a  bottle-nosed  dolphin.      (U.   S.   Fish  Com.) 

the  shallow  seas  includes  the  greater  part  of  the  more 
familiar  forms  such  as  the  fishes,  molluscs,  crabs,  lobsters, 
corals,  sea  urchins,  star  fishes,  porpoises,  and  seals,  all  in 
great  number  and  variety.  In  places  there  is  also  prolific 
vegetable  life  which  is  necessary  for  the  support  of  the 
animal  life.     The  shallower  portions  of  the  ocean,  known 


THE  OCEAN 


193 


Fia.  139.  Giant  squid,  Port  Otway,  W.  Patagonia.  It  frequents  the 
shallow  water  near  shore  in  cold  climates.  It  is  a  large  and  voracious 
animal  belonging  to  the  same  class  as  the  devil  fish.      (U.  S.  Fish  Com.) 

13 


194  PHYSICAL  GEOGRAPHY 

as  banks,  such  as  the  Grand  Banks  off  the  coast  of  New- 
foundland, are  frequented  by  vessels  from  distant  lands 
for  the  cod  and  other  fishes  which  swarm  here  in  great 
numbers.  The  coral  reefs,  which  grow  only  in  shallow 
water,  teem  with  multitudes  of  living  forms.  Indeed, 
there  are  few  places  where  life  is  more  prolific  than  on  a 
coral  reef.  The  littoral  or  shore  life  includes  the  eel  grass, 
marsh  grass,  mangrove,  and  other  plants,  besides  the  great 
variety  of  animal  life.     (Figs.  138  and  139.) 

Deep  sea  life.  The  life  on  the  deep  ocean  bottom  is 
quite  meagre,  consisting  of  a  few  strange  and  fantastic 
forms.  The  conditions  are  unfavorable  for  abundance  of 
life,  as  it  is  everywhere  dark,  cold,— almost  at  the  freezing- 
point— and  there  is  enormous  pressure  from  the  great 
depth  of  water. 

Despite  the  dreary,  monotonous,  and  undesirable  con- 
ditions of  deep  sea  bottom,  there  is  a  growth,  scanty  it  is 
true,  of  living  forms  over  a  considerable  portion  of  it. 
There  is  probably  no  vegetation,  as  that  requires  some  sun- 
light, and  hence  the  animals  of  the  sea  bottom  are  de- 
pendent for  their  food  supply  on  the  remains  of  the  sur- 
face forms  which  sink  to  the  bottom. 

Many  of  the  deep  sea  forms,  as  they  are  brought  up  In 
dredges,  perish  as  soon  as  they  reacli  the  surface  because  of 
the  great  change  in  pressure.  At  a  depth  of  20,000  feet  there  is 
a  pressure  from  the  overlying  water  of  over  600  tons  per 
square  foot.  The  animals  of  the  deep  sea  resist  this  pressure 
in  the  same  way  that  we  resist  the  pressure  of  the  atmosphere, 
namely,  by  having  a  corresponding  pressure  on  the  inside.  When 
they  are  somewhat  suddenly  released  from  the  great  external 
pressure,  before  there  is  a  proper  adjustment  of  the  internal 
pressure,  the  result  is  generally  disastrous. 

Pelagic  life.  The  life  on  and  near  the  surface  of  the 
open  sea— the  pelagic  life— is  nearly  everywhere  abundant, 
but  especially  so  in  the  tropical  regions.     There  is  a  great 


THE  OCEAN  195 

variety  as  well  as  quantity  of  forms,  varying  in  size  from 
the  multitudes  of  microscopic  plants  and  animals  (see  sec. 
158)  to  the  huge  whales  which  feed  upon  them. 

The  floating  vegetation  of  the  sargasso  seas  attract  many 
forms  of  animal  life  which  make  it  their  feeding  ground, 
and  they  in  turn  form  food  for  carnivorous  forms  which 
are  thus  attracted. 

The  different  forms  of  pelagic  life,  including  the  floating 
vegetation,  the  microscopic  animals,  whales,  fishes,  and  other 
free  swimming  forms,  are  common  also  in  the  surface  waters  of 
the  shallow  water  areas  of  the  continental  shelf.  The  range  of 
the  different  species  over  the  surface  of  the  ocean  is  limited  by 
the  changes  in  temperature  and  not  by  the  depth  of  the  water. 

The  great  part  of  the  pelagic  life  lies  on  or  close  to  the  sur- 
face of  the  ocean,  between  which  and  the  sparsely  inhabited  sea 
bottom  is  the  great  bulk  of  the  oceanic  waters — dark,  cold, 
dreary,  monotonous  zones — great,  watery  desert  areas,  almost 
barren  of  life. 

161.  Economic  Features  of  the  Ocean.— "Old  Ocean's 
gray  and  melancholy  waste,"  like  many  other  poetical  ex- 
pressions, is  very  misleading  and  untrue  if  we  attempt  to 
apply  it  literally.  It  is  the  greatest  and  by  far  the  best 
of  all  our  highways,  which,  besides  being  free,  extends  to 
nearly  every  nation  and  serves  to  unite  the  civilized  coun- 
tries into  a  great  commercial  family. 

The  ocean  makes  the  land  habitable  by  furnishing 
moisture  and  tempering  the  climate.  It  carries  the 
warmth  of  the  tropical  sun  to  the  temperate  and  polar 
regions  and  in  turn  transports  the  cold  of  the  poles  to- 
wards the  equator.  It  is  the  chief  factor  in  the  circula- 
tion of  moisture  through  the  atmosphere. 

An  important  part  of  the  food  supply  of  the  world 
comes  from  the  ocean.  Probably  most  important  of  all 
are  the  fishes  of  many  kinds.  Make  a  list  of  the  names  of 
all  the  fishes  that  you  know  are  taken  from  the  ocean  for 


196  PHYSICAL  GEOGRAPHY 

food.  Besides  the  fishes  there  are  oysters,  clams,  lobsters, 
crabs,  shrimps,  walruses,  polar  bears,  whales,  porpoises, 
and  seals.  Other  important  products  are  pearls,  coral, 
sponges,  shells,  salt,  and  seaweed.  In  some  countries  sea- 
weed is  used  extensively  for  food.  What  other  uses  has 
it?  It  is  estimated  that  the  annual  value  of  the  food 
products  taken  from  the  sea  is  not  less  than  $500,000,000. 


REFERENCES 

Pillsbury,  The  Gulf  Stream,  the  Annual  Report,  U.  S.  Coast 
Survey,  1890. 

Page,  Ocean  Currents,  Monthly  Weather  Review,  August, 
1902,  p.  397. 

Davis,  Winds  and  Ocean  Currents,  Journal  of  School  Geog- 
raphy, Vol.  II,  1898,  p.   16, 

Everman,  Strange  Fishes  of  the  Deep  Sea,  The  World  of  To- 
Day,  June  and  September,  1902. 

Goode,  Deep  Sea  Fishes. 

Thomson,  The  Depths  of  the  Sea,  and  the  Voyage  of  the 
Challenger;   Macmilla^n  Company. 

Sigsbee,  Deep  Sea  Sounding  and  Dredging,  Washington,  D.  C, 
1880. 

Tanner,  Deep  Sea  Exploration,  U.  S.  Fish  Commission,  Wash- 
ington, D.  C. 

Flint,  Oceanography  of  the  Pacific,  Bull.  No.  55,  U.  S.  Natl. 
Museum. 

Agassiz,  Three  Cruises  of  the  Blake,  2  vols.,  Houghton, 
Mifflin  &  Company,  Boston,  1888. 

Maury,  Physical  Geography  of  the  Sea. 

Littlehales,  Marine  Hydrographic  Survey  of  the  Coasts  of 
the  World,  8th  Rept.  Int.  Geog.  Cong.  Washington, 
1904,  p.  576. 


CHAPTER  VI 
SHORE  LINES 

162.  The  shore  line  is  the  line  where  land  and  sea 
areas  meet ;  the  line  above  and  below  which  there  is  an 
abrupt  change  in  the  living  forms  and  sometimes  a  very 
sudden  change  in  the  topographic  forms.  It  is  on  and 
near  the  shore  where  great  numbers  of  plant  and  animal 
remains  are  buried  and  preserved  in  the  shore  sediments, 
hence  furnishing  a  key  to  the  life  of  the  geological  period. 
Thus  by  studying  the  fossil  forms  in  the  shore  deposits  of 
the  past,  one  gains  a  knowledge  of  the  life  of  previous 
ages.  Animals  and  plants  living  during  one  geological 
period  are  different  from  those  of  the  preceding  and  fol- 
lowing periods ;  hence  the  fossil  remains  in  the  ancient 
shore  deposits  indicate  the  geologic  age  in  which  the  de- 
posits were  formed.     (See  fig.  140.) 

163.  Topography.— The  shore  line  has  a  topography 
peculiarly  its  own.  The  bars,  beaches,  projections,  in- 
dentations, and  deposits  are  in  many  respects  different 
from  anything  inland  or  on  the  sea  bottom.  A  knowl- 
edge of  shore  forms  and  shore  features  aids  greatly  in  the 
study  of  the  geography  of  the  past,  the  distribution  of 
land  and  sea  areas  in  the  past  ages. 

Besides  the  lessons  taught  by  the  shore  features  and  de- 
posits, there  is  a  wonderful  inspiration  in  standing  on  the  shore 
and  contemplating  the  vastness  of  the  ocean  and  the  ceaseless 
beat  and  roll  of  the  waves.  It  appeals  to  man  in  his  con- 
templative moods  in  much  the  same  degree,  but  in  a  different 
manner  from  that  of  the  grandeur  of  the  mountains,  or  the  soli- 
tude of  the  desert. 

197 


198 


PHYSICAL  GEOGRAPHY 


Like  the  river,  the  mountain,  the  plain,  and  other  natural 
features,  the  shore  line  appears  to  follow  a  more  or  less  regular 
cycle  of  change  and  have  a  life  history  of  its  own.  But  because 
it  is  more  sensitive  to  slight  changes  of  elevation  or  depression 
and  probably  more  subject  to  such  changes,  the  shore  cycle  is 
liable  to  be  interrupted  or  broken  so  frequently  that  its  exist- 


iiu.  140.  Bodkin  Point,  Md.  A  buried  cypress  forest  ao-w  being  uncovered 
by  the  waves.  Remains  of  both  the  ancient  forest  and  the  living  one  are 
being  buried  in  the  sands  of  the  present  beach  to  tell  the  story  in  future 
ages.      (Maryland  Geological   Survey.) 


ence  is  commonly  overlooked  and  many  persons  thus  fail  to  read 
the  past  history  of  a  shore  by  studying  its  present  features.  Yet 
the  phenomena  on  any  shore  line,  if  properly  interpreted,  indi- 
cate many  of  the  changes  it  has  undergone  and  will  undergo. 

164.  Shore  Erosion. — The  erosion  on  the  shore  line 
is  done  mostly  by  the  waves,  which  erode  the  rocks  much 
as  the  rain  and  the  streams  do  the  upland.  The  corrad- 
ing  action  of  the  waves  is  aided  by  the  other  weathering 


SHORE  LINES  199 

agencies,  such  as  the  frost,  wind,  chemical  action,  plants, 
and  animals.  The  tides  assist  the  wave  action  by  lifting 
and  lowering  them  to  new  points  of  attack. 


Fig.  141.  Pacific  Coast  on  the  17-mile  drive  at  Monterey,  Cal.  Showing 
breakers  and  the  effect  of  the  waves  on  hard  rocks.  Why  are  the  rocks 
on  the  beach  more  rounded  than  those  out  in  the  water?  (Detroit  Pub. 
Co.) 

The  active  work  of  the  waves  is  done  mostly  during  a 
storm,  or  by  the  irregular  heavy  earthquake  (so-called 
tidal)  waves  that  occasionally  deluge  a  coast.  The  storm 
waves  on  the  coast  correspond  in  their  corrading  effect  to 
the  freshet  in  the  river. 

165.  Effect  of  Storm  Waves.— The  work  of  the  storm 
waves  is  confined  largely  to  a  vertical  range  from  about 
50  feet  below  tide  to  100  feet  or  so  above  tide.  On  this 
comparatively  narrow  vertical  belt  their  force  is  frequent- 


200  PHYSICAL  GEOGRAPHY 

ly  terrific  and  their  work  is  accomplished  in  several  ways: 

(1)  The  impact  of  the  boulders,  shingle,  and  sand,  that 
are  picked  up  by  the  waves  and  hurled  against  the  rocks 
with  great  force,  loosens  more  material  from  the  cliff  and 
breaks  and  grinds  up  that  already  loosened.  On  the 
Bahama  Islands,  blocfe  weighing  twenty  tons  have  been 
hurled  by  the  waves  125  feet  from  the  shore  and  25  feet 
above  high  water. 

(2)  The  boulders,  gravel  and  sand  torn  from  the  cliff 
are  rolled  up  and  down  the  sloping  beach,  being  thus  worn 
smaller,  while  the  finer  material  is  swept  back  into  deeper 
water, 

(3)  The  spray  acts  both  mechanically  and  chemically. 
On  the  coast  of  Scotland,  light-house  windows  have  been 
broken  at  the  height  of  300  feet.  The  spray  dashing 
against  the  rocks,  .high  above  the  reach  of  the  waves,  as- 
sists in  the  chemical  disintegration  of  the  rock  constituents. 

(4)  The  hydrostatic  pressure  of  the  water,  and  the 
compression*  and  expansion  of  the  air  driven  by  the  waves 
into  the.  crevices  and  caves  along  the  shore,  are  agents  of 
disintegration. 

166.  Tidal  Waves.— True  tidal  waves  are  not  very  powerful 
eroding  agents  on  the  open  shore,  but  in  bays,  estuaries,  and 
river  channels  they  are  often  quite  active.  The  hore  or  pororoca 
of  the  Amazon,  the  mascaret  of  the  Seine,  and  similar  waveg 
in  other  rivers,  are  great  tidal  waves  which  sweep  with  high 
velocity  up  the  river  channel,  destroying  and  tearing  away  the 
material  along  the  banks,  proving  destructive  to  boats  on  the 
river  as  well  as  to  property  on  the  shore.  The  so-called  tidal 
wave  that  destroyed  so  much  property  and  flooded  such  ex- 
tensive land  areas  at  Galveston  in  1902  was  produced  by  the 
hurricane  which  it  accompanied  and  was  in  no  way  related  to 
the  tide. 

167.  Earthquake  Waves.— Sometimes  a  coast  or  a 
portion  of  a  coast  is  visited  by  a  large  and  very  destruc- 


SHORE  LINES 


201 


tive  wave  frequently  but  wrongly  called  a  tidal  wave,  as 
it  in  nearly  all  eases  accompanies  an  earthquake  or  vol- 
canic disturbance.  Such  waves  are  not  destructive  on 
the  open  sea,  where  they  pass  as  low  waves  of  great  length 
and  often  unperceived;  but  as  they  approach  the 'shore 


Fig.  142.  Shore  of  Lake  Ontario,  near  Oswego,  N.  Y.  A  wave-cut  cliff  which 
is  being  undercut  and  pushed  back  by  the  eroding  and  undermining  action 
of  the  waves.      (Oliphant. ) 


and  drag  the  bottom,  the  water  begins  to  pile  up  and  rush 
in  on  the  shore  in  an  overwhelming  flood  that  causes 
enormous  loss  of  life  and  property  and  frequently  modi- 
fies the  shore-line  beyond  recognition. 

168.  Transportation  and  Deposition  Along  the  Shore. 
— The  volume  of  water  that  is  carried  in  on  the  shore  on 
the  crest  of  the  waves  is  returned  along  the  bottom  in  the 


202 


PHYSICAL  GEOGRAPHY 


undertow   which   carries   back  with   it   the   fine   material 
formed  by  the  grinding  of  the  shingle  on  the  beach.    This 


7m 


J  \ 


^ 


^'x  5 


^f 


^■f 

m       r~w^ 

V^^^^^l 

^^^B 

\  » ' 

u /V^^^^l 

HhBBT 

'//  '^  fl^^H 

W^ 

i 

:  ': 

o 

■:•  a 


I 


material  is  carried  out  from  the  beach  and  deposited,  in 
the  order  of  the  size  of  the  fragments,  with  the  coarsest 


SHORE  LINES 


203 


nearest  the  shore  and  the  finest  farthest  out.  Where  there 
are  currents  at  or  near  the  shore,  they  affect  the  distribu- 
tion of  the  material  by  moving  it  along  the  beach  or  car- 
rying it  further  out  in  the  sea. 

Besides  that  carried  out  by  the  undertow  and  the  cur- 
rents, there  is  frequently  a  movement  of  material  along 
the  beach  by  the  waves  and  the  shore  currents  produced 
by  the  waves  where  they  strike  the  shore  obliquely.  It  is 
in  this  way  that  bars,  cusps,  spits,  hooks,  and  wall  beaches 
are  formed.     (See  sections  171  to  173.) 

169.  Topographic  and  Structural  Features  of  Shore 
lines. — Where  the  waves  beat  against  a  rocky  shore,  they 


Fig.  144.  Diagram  showing  a  wave-cut  and  wave-built  terrace  on  the  shore 
line.  Dotted  line  A,  position  of  the  former  shore.  B,  the  portion  of  the 
land  cut  away  by  the  waves.  C,  the  wave-cut  terrace.  D,  the  wave-built 
terrace  composed  of  the  fragments  worn  from  the  cliff  by  the  waves  and 
carried  back  by  the  undertow. 


cut  away  the  rocks  near  the  water-line  and  the  shore-line 
is  carried  into  the  land  by  the  wearing  away  of  the  rocks, 
forming  the  wave-cut  terrace.  As  the  terrace  is  cut  back 
into  higher  land  the  latter  is  undermined  and  a  vertical 
or  overhanging  shore  cliff  is  formed,  that  recedes  as  the 
water  undercuts,  by  the  upper  portion  falling  down,  to 
be  ground  up  and  carried  away. 

The  material  ground  up  by  the  waves  at  the  base  of 
the  cliff  is  carried  back  into  deeper  water  by  the  under- 


204 


PHYSICAL  GEOGRAPHY 


Fig.  145.  A  chimney  rock  formed  by  shore  erosion,  Port- 
land, England.  A  remnant  left  by  the  wearing  away 
of  the  surrounding  rocks  by  the  waves. 

tow  and  deposited  beyond  the  edge  of  the  wave-cut  ter- 
race, making  the  wave-built  terrace.     (See  figs  142  to  144.) 
Chimney  Rocks.     As  the  waves  and  the  gravel  beach 
cut  into  the  rock  cliff,  some  portions  of  it  are  left  stand- 


SHORE  LINES  205 

ing,  while  the  waves  cut  down  and  carry  away  the  material 
on  all  sides.  These  remnants  commonly  include  portions 
between  joint-planes  and  are  generally  for  that  reason 
quite  angular,  sometimes  rectangular.  From  their  form 
they  are  commonly  called  chimney  rocks  (See  fig.  145.) 


Fig.  146.     Remnant  of  a  sea  cave  in  a  chimney  rock  on  the  California 
coast,  near  San  Diego.      (J.  C.  Branner.) 

Efect  of  dikes  on  the  shore  line.  Sometimes  an  intrusive  dike 
of  igneous  rock  (see  sec.  236)  is  more  resisting  to  the  weather 
than  the  surrounding  rock  which,  crumbling  away  more  rapidly, 
leaves  the  dike  standing  above  the  surface  often  like  a  great 
row  of  cord-wood  jutting  out  on  the  beach,  sometimes  out  into 
the  water.  In  other  places  the  igneous  dike  weathers  more 
rapidly  than  the  surrounding  rock,  and  thus  forms  great  chasms 
extending  back  into  the  cliff  where  the  dike  material  has  been 
torn  out  by  the  waves.  Sometimes  a  chasm  of  this  kind  is  cut 
across  a  headland,  separating  the  end  from  the  mainland  and 
thus  forming  an  island.  There  is  one  place  on  the  north  shore 
of  Lake  Superior  where  from  a  boat  one  may  see  fourteen  of 
these  dark-colored  dikes  cutting  the  light-colored  granite  rock 
within  a  distance  of  less  than  a  mile. 

In  some  places  on  a  narrow  neck  of  land  the  bottom  portion 
is    cut  through   first,   leaving  the   upper   portion    standing  as    a 


206  PHYSICAL  GEOGRAPHY 

natural  bridge.     Several  such  bridges  occur  on  the  coast  of  Cal- 
ifornia.    (See  figs.  146  and  148). 

170.  Sea  Caves. — Sea  caves  are  formed  when  the 
waves  undercut  the  cliff  more  rapidly  at  one  point.  They 
commonly  begin  at  a  soft  place  in  the  rock,  a  fissure,  or  a 


Fig.  147.  Deep  chasm  formed  by  a  dike  of  igneous  rock  disintegrating 
more  rapidly  than  the  harder  wall  rock.  This  chasm  on  the  shore  of  a 
small  lake  in  the  Adirondacks  was  not  formed  by  the  waves  but  is 
similar  to  many  that  were  so  formed  along  the  shores  of  the  Great 
Lakes  and  the  ocean.      (S.  R.  Stoddard.) 

joint-plane.  Generally  such  caves  are  not  very  long,  be- 
cause as  soon  as  the  opening  is  made,  the  inrushing  wave 
blocks  the  mouth  of  the  cave,  when  the  air  is  compressed 
and  acts  as  a  cushion  to  protect  the  rocks  within  from  the 
blow  of  the  wave.  The  compression  of  the  air  and  the 
sudden  expansion  when  the  wave  recedes  may  tear  loose 
some  blocks  and  thus  enlarge  the  cave.     Sometimes  the 


SHOKE  LINES 


207 


land  side  of  the  cave  is  worn  away,  leaving  a  natural  bridge 
on  the  shore.     (See  figs  148  and  149.) 


!PiG.  148.  Natural  bridge  formed  by  action  of  the  waves  on  the  shore  of  the 
Pacific  near  Santa  Cruz,  Cal.  The  top  of  the  bridge  is  part  of  the  uplifted 
coastal  plain  that  borders  the  shore.      (J.  C.   Branner.) 


Fig.  149.     Shore  of  the  Pacific  Ocean  at  La  Jolla,  Cal.,  showing  sea  caves  and 
other  evidences  of  wave  erosion.      (J.  C.   Branner.) 


OF  THE 

UNIVERSITY 


208  PHYSICAL  GEOGRAPHY 

Whistling  caves  and  blow  holes  are  formed  in  the  sea  caves, 
where  an  opening  occurs  in  the  roof  that  permits  the  escape  of 
the  imprisoned  air,  which  rushes  out  often  with  great  violence, 
producing  a  noise  something  like  a  steam  whistle.  Sometimes 
these  caves  are  so  shaped  that  not  only  the  air  but  a  column  of 
water  is  forced  out  through  an  opening  at  the  top,  forming  the 
spouting  cave.     (Fig.  150). 


Fig.  150.  Spouting  Cave  in  the  ice  on  the  shore  of  Lake  Ontario  at  Oswego, 
N.  Y.  Somewhat  similar  caves  occur  in  the  rocks  in  places  on  the  ocean 
shore.      (M.  S,  Lovell.) 

171.  The  Beach.— The  beach  varies  in  character  at 
different  points.  In  the  smaller  coves  on  the  headlands 
and  on  the  bold,  rocky  shores,  there  are  great  accumula- 
tions of  boulders  and  gravel.  At  the  head  of  larger  bays 
and  along  low,  shores  the  beach  is  covered  with  sand  or 
mud.  The  character  of  the  material  on  the  beach  largely 
depends  upon  whether  there  is  a  shore  cliff,  the  kind  of  rock 
in  the  cliff,  the  shape  of  the  clift',  the  form  of  the  shore 
adjoining  the  cliff,  and  the  direction  of  the  winds. 


SHORE  LINES 


209 


The  shingle  beaches  are  formed  at  the  base  of  rock 
cliffs  where  the  fragments  from  the  cliff  are  ground  up 
by  the  waves.  Where  the  incoming  waves  strike  the  shin- 
gle beach  obliquely,  the  material  is  moved  along  the  beach 
beyond  the  end  of  the  cliff,  forming  the  wall  heach  or 
travelling  beach,  which  frequently  extends  across  the  mouth 
of  a  stream  and  forms  a  lake  or  causes  the  stream  to  shift 
its  course  beyond  the  end  of  the  shingle  wall.  The  wall 
beach  is  formed  wherever  the  material  is  eroded  from  the 
cliff  faster  than  it  is  carried  back  into  deep  water. 


FiQ.  151,     A  hooiced  sand  spit  at  Dutch  Point  on  Lake  Michigan. 
Geol.   Survey. ) 


172.  Spit,  Hook.— Winds  and  shore  currents  trans- 
fer materials  along  the  beach,  and  frequently,  where  there 
is  a  bend  in  the  shore  or  a  change  of  direction,  the  shore 
current  may  be  deflected  out  into  deep  water  and  the 
beach  accumulation  be  extended  out  from  the  shore  as  a 
point  or  arm,  which  is  known  as  a  spit.  When  the  point 
is  recurved  it  forms  a  hook,  or  hooked  spit.     The  curving 

14 


210  PHYSICAL  GEOGRAPHY 

of  the  hook  is  commonly  due  to  the  action  of  another  cur- 
rent at  an  angle  to  the  first  one.  The  second  current  may- 
be temporary,  due  to  a  violent  storm;  and  after  it  ceases, 
the  spit  may  continue  in  the  direction  first  taken  until  it 
meets  another  storm,  where  another  hook  may  form,  mak- 
ing in  this  way  a  series  of  hooks  or  barbs  on  the  same  spit. 
Spits  are  also  formed  in  quiet  waters  between  two  cur- 
rents which  carry  sediment.     (Figs.  151  and  152.) 


Pig.   152.     A  sand  spit  forming  a  bar   across  mouth  of  Floyd's   Creek,   Mary- 
land.     (Maryland  Geological  Survey.) 

173.  Bars.— The  spit  may  form  at  the  headland  at 
the  opening  of  a  bay,  or  it  may  form  along  the  sides  of  a 
bay.  In  time,  if  it  is  not  checked  by  a  strong  river  or 
tidal  current  through  the  bay,  it  may  extend  entirely 
across,  joining  the  land  on  the  opposite  side,  when  it  is 
called  a  har,  sometimes  a  hay  bar  to  distinguish  it  from 
bars  formed  in  other  ways. 


SHORE  LINES 


211 


Small  islands  along  a  shore  are  frequently  tied  or  joined  to 
the  shore  by  a  bar  which  may  have  started  as  a  spit  from  the 


Fig.  153.  Tie-bars  connecting  the  mainland  of  Italy  with  Monte  Argen- 
tario  Island.  (See  Boston  Bay,  Mass.,  Sheet  U.  S.  Top.  Atlas  for 
similar  example  on  New  England  Coast.) 


island  or  from  the  shore  or  from  both  places.  There  may  be 
either  one  or  two,  sometimes  more  of  these  bars  tying  the  island 
to  the  shore.     They  are  formed  where  the  islands  lie  near  shore 


212 


PHYSICAL  GEOGRAPHY 


and  there  is  sufficient  rock  waste  from  them.     They  are  some- 
times called  tie-bars  because  they  tie  the  island  to  the  shore. 

174.  Barriers.— Where  there  is  a  stretch  of  shallow 
water  off  shore,  there  is  sometimes  a  violent  agitation  of 
the  bottom  sand  and  mud  by  waves  which  form  the  break- 
ers at  some  distance  out  from  the  shore.  The  meeting, 
in  these  muddy  waters,  of  the  waves  from  the  sea  on  one 


foreland. 


of  Lake  Champlain,  Y/ 
t»uiall  sand  barrier,  lagoon 
(H.  M.   Brock.) 


iiiia    loiiy:    sand    spil    or    uuspate 


side  and  the  undertow  from  the  land  on  the  other  side, 
checks  the  velocity  of  each,  causes  a  deposit,  and  builds 
up  an  off-shore  ridge  or  bar,  which  in  time  reaches  the 
surface,  above  which  it  is  built  by  the  waves  and  wind. 
Such  an  off-shore  bar  is  called  a  harrier.  The  shallow 
water  or  lagoon  behind  the  barrier  is  in  time  filled  by  the 
sediment  carried  in  by  the  rivers,  the  sand  and  dust  car- 
ried in  by  the  winds,  both  aided  materially  by  the  accu- 


SHORE  LINES 


213 


mulations  of  vegetable  and  animal  matter.  After  the 
filling  of  the  lagoon,  the  former  barrier  becomes  the  beach 
of  the  new  shore  line  and  another  barrier  develops,  in  this 
way  extending  the  land  area  into  the  sea.  A  barrier  beach 
is  formed  where  the  water  is  too  shallow  for  the  waves  to 
attack  the  shore.  (For  description  of  the  coral  barrier 
reef  see  sec.  181). 

175.     Shore  Terraces.— Terraces  are  formed  along  the 


Fig.  155.  Sand  terraces  on  the  north  shore  of  Lake  Superior.  Drawn 
to  scale  from  barometric  measurements  by  the  author.  The  terraces 
are  known  to  the   Indians   as  "Manabozho's  stair  steps." 


shore,  by  the  elevations  of  the  land  which  carry  the  first 
beach  above  high  water  and  expose  a  new  surface  on 
which  another  beach  is  formed.  The  elevated  beaches 
form  in  some  places  boulder  terraces,  in  others  sand  or 
gravel  terraces,  in  others  bed-rock  or  wave-cut  terraces, 
(See  figs  155  and  155a.) 

176.  Irregular  Shore  Lines.— An  irregular  shore  line 
is  generally  caused  by  a  migration  of  the  shore  line  land- 
ward, due  either  to  subsidence  of  the  land  area  or  to  the 
erosive  action  of  the  waves.  The  beating  of  the  waves  on 
a  new  shore  line  will  first  wear  away  the  softer  rocks,  or 
the  more  exposed  ones,  forming  indentations  or  bays,  with 


214 


PHYSICAL  GEOGRAPHY 


the  harder  rocks  standing  out  as  headlands  and  promon- 
tories. Irregularities  in  the  rocks  on  the  headlands  cause 
caves,  natural  bridges,  chimney  rocks  and  islands.  (See 
figs.  156,  157,  146,  148  and  149.) 


am.  155a.  Shore  terraces  on  the  moufctain  bordering  Great  Salt  Lake, 
Utah.  The  terraces  mark  former  shore  Ijnes  of  the  extinct  Lake  Bonne- 
ville. Notice  the  desert  vegetation  in  the  foreground.  Salt  Lake 
visible  in  the  background  on  the  right.      (D.  T.  McDougal.) 

The  most  irregular  shore  lines  are  produced  by  the  sinking 
of  the  land  and  the  advance  of  the  sea,  in  which  case  the  sea 
extends  up  the  valleys,  sometimes  long  distances,  forming  bays, 
estuaries  or  fiords,  that  is,  drowned  valleys.  The  hills  bordering 
the  valleys  form  headlands  on  the  submerged  area.  What  were 
monadnocks  and  peaks  before  submergence,  become  islands  along 


SHORE  LINES 


215 


the  new  shore.  In  the  drowning  of  the  lower  portion  of  the 
valleys  the  rivers  are  dismembered  and  the  tributaries  flow  di- 
rectly into  the  arm  of  the  sea.  (Study  the  Boothbay  sheet  of 
the  U.  S.  Top.  Atlas.) 


Fig.  156.  Irregular  shore  on  the  Oregon  coast,  formed  by  wave  erosion.  The 
more  resistant  rocks  form  the  headlands  and  islands,  which  are  the 
remnants  of  the  land  area  cut  away  on  each  side  by  the  waves.  (J.  E. 
Kirkwood.) 


177.  Regular  Shore  Lines.— Regular  shore  lines  are 
formed  in  several  ways:  (1)  By  the  uplift  of  the  land, 
which  causes  the  shore  line  to  miove  seaward  from  the  old 
land  border  on  to  the  newly  uplifted  coastal  plain,  pro- 
ducing a  quite  regular  coast-line.  This  regularity  of  shore 
line  is  due  to  the  smoothness  of  the  sea  bottom. 

(2)  By  the  building  of  sand  reefs  parallel  to  the 
shore,   causing  a  movement  of  the  shore  outward  to  the 


216 


PHYSICAL  GEOGRAPHY 


SHORE  LINES  217 

reef.     This  filling  in  of  the  lagoon  and  marshes  inside  the 
bar,  removes  the  inequalities. 

(3)  By  the  waves  cutting  off  the  headlands  and  filling 
up  the  bays,  thus  making  a  more  regular  coast  line. 

(4)  By  delta  deposits  at  the  mouth  of  a  river  flowing 
into  a  bay  finally  filling  the  bay  and  thus  straightening 
that  portion  of  the  coast.  The  extension  of  the  bay-bars 
across  the  bay  and  the  tie-bars  between  the  islands  and 
the  mainland  tend  to  straighten  the  coast  line. 

SHORE  LINES   MODIFIED   BY  LIVING   FORMS 

The  shore  line  is  sometimes  greatly  modified  by  tKe 
accumulation  of  organic  matter,  both  vegetable  and  ani- 
mal. 

178.  Vegetable. — In  tropical  regions  one  of  the  most 
important  plants  that  affect  the  shore  line  is  the  Mangrove 
tree,  one  of  the  very  few  trees  that  flourishes  in  salt  water. 
The  seed  often  sprouts  while  still  attached  to  the  branch, 
and  sends  forth  a  long  radicle  or  root  stem  which  fre- 
quently extends  to  the  mud  at  the  bottom,  takes  root  and 
from  the  top  a  new  stem  or  trunk  is  formed.  From  the 
trunk  many  spreading  roots  extend  to  the  bottom,  and  many 
branches  form  at  the  top,  some  extending  downward  to 
start  new  roots  and  new  trunks,  until  a  single  parent  tree 
is  surrounded  by  a  small  grove.  Some  of  the  fruit  drops 
off  and  floats  away  on  the  water  with  the  long  radicle  ex- 
tending towards  the  bottom  until  it  finally  becomes  at- 
tached to  the  bottom  mud  and  starts  a  new  tree  and  a  new 
grove. 

The  network  of  roots  catches  and  holds  drift  material 
and  mud,  until  a  solid  embankment  is  built  up.  The  shal- 
low lagoon  between  it  and  the  mainland  in  time  fills  with 
accumulated  vegetable  and  animal  matter  and  mud   de- 


218  PHYSICAL  GEOGRAPHY 

posits.  A  continuation  of  this  process  extends  the  shore 
line  seaward  wherever  the  building  up  is  faster  than  the 
destruction  by  the  waves.  The  shore  plain  built  in  this 
way  is  apt  to  be  wet  and  marshy.  The  mangrove  is  very 
abundant  on  the  coast  of  Florida.     (See  fig.  158.) 


Fig.  158.  Mangrove  trees  on  the  shore  of  a  lagoon  at  low  tide.  Gilbert  Group 
of  coral  islands  in  the  Pacific  Ocean.  The  area  is  covered  by  the  sea  at 
high  tide.      (U.  S.  Fish  Com.) 

Eel  and  marsh  grass.  The  mangrove  does  not  grow  north  of 
Florida,  but  the  low-lying  plains  bordering  the  shallow  water 
along  the  northern  shore  of  the  United  States  are  extended  sea- 
ward in  a  somewhat  similar  manner  by  another  kind  of  vegeta- 
tion. The  eel  grass  grows  over  the  shallow  bottom  below  low 
tide,  where  it  acts  as  a  trap  to  catch  the  mud  stirred  up  by  the 
waves,  until  the  bottom  is  raised  to  low  tide  surface-level,  when 
the  marsh  grass  takes  possession  and  aids  in  the  upbuilding 
process  up  to  or  sometimes  above  high  tide.  The  repetition  of 
this  process  causes  the  extension  of  the  marsh  grass  plains 
seaward. 

179.  CoraL—One  of  the  most  prominent  of  all  the 
land  builders  along  the  shores  is  the  coral  polyp,  an  ani- 


SHORE  LINES 


219 


mal  which  secretes  carbonate  of  lime  that  it  extracts  from 
sea  water.  It  grows  in  such  multitudes  that  even  though 
a  single  coral  polyp  secretes  but  a  small  quantity  of  lime 


t'' 

^^^ 

ly       ^Xr 

^^  *«^^ 

^ 

^^^^? 

i 

mm 

m 

^»^ 

Fig.   159.     A  coral  colony  showing  the  polyps  opened.     The  polyps 
somewhat    resemble    flowers.      (Smith.     Inst.) 


the  aggregate  is  something  astounding.  The  Great  Bar- 
rier Reef  off  the  coast  of  Australia  is  more  than  1,000 
miles    long   and    contains    a   mass   of   limestone   probably 


220  PHYSICAL  GEOGRAPHY 

equal  to  any  of  the  great  limestone  beds  extending  through 
the  central  and  eastern  United  States. 

The  reef-building  coral  flourishes  only  in  tropical  seas  where 
the  winter  temperature  of  the  waters  does  not  fall  below  68  de- 
grees F.  It  does  not  grow  above  the  surface  of  the  salt  water 
at  low  tide  nor  at  depths  greater  than  one  hundred  feet.    It  does 


Fig.    160.     Coral    Head    or    Mushroom.      Beach    north    of    Hepuhepuama, 
Makemo  atoll,  in  the  Pacific  Ocean.      (U.  S.  Fish  Com.) 

not  grow  in  muddy  waters,  hence  is  not  found  at  the  mouths  of 
rivers.  It  grows  best  in  waters  that  are  violently  agitated  by 
the  waves  and  currents,  hence,  it  is  not  found  in  great  abun- 
dance inside  the  atolls,  but  flourishes  on  the  outside  in  the  midst 
of  the  surf  and  breakers.  The  reason  for  this  is  that  it  needs  a 
constant  supply  of  food,  oxygen,  and  carbonate  of  lime,  which 
is  soon  exhausted  in  the  lagoon,  but  is  constantly  renewed  by  the 
moving  waters  in  the  breakers. 


SHORE  LINES 


221 


Thick  coral  beds.  While  the  coral  does  not  grow  at 
depths  greater  than  100  feet,  some  of  the  reefs  appear  to 
be  several  thousand  feet  deep ;  at  least  the  dredge  brings 
up  dead  coral  from  that  depth  off  the  shore  of  the  reefs. 
This  is  accounted  for  in  two  ways :  ( 1 )  The  coral  grow- 
ing outward  from  the  reef  at  the  top  forms  overhanging 
masses  which  break  off  from  their  weight  or  are  broken 
off  by  the  weaves  and  slide  down  the  steeply  inclined 
sea-bottom  into  the  deep  water;  or  (2)  the  bottom  sub- 
sides  as   the  coral   is   growing   and   the  coral   that   grows 


Fig.  161.  Illustrating  the  development  of  an  atoll  from  a  fring- 
ing coral  reef.  ss,  former  sea  level.  F."  fringing  reef, 
s'  s',  subsequent  level  of  the  sea  after  sinking  of  the  island. 
BB,  barrier  reef,  s"  s",  sea  level  after  further  subsidence 
when  AA  is  a  coral  atoll  surrounding  a  lagoon.      (Darwin.) 


near  the  surface  is  carried  down  into  deep  waters  by  the 
sinking  of  the  bottom.  This  may  continue  indefinitely 
without  killing  the  coral  at  the  top,  providing  the  sinking 
does  not  take  place  more  rapidly  than  the  coral  grows.  It 
may  be  slower,  but  not  faster.     (See  fig.  161.) 

180.  Coral  reefs. — The  coral  deposits  attached  to  the 
shore  form  fringing  reefs,  such  as  those  on  the  Bahama 
Islands.  Those  that  occur  out  from  the  shore  at  dis- 
tances varying  from  a  fraction  of  a  mile  to  several  miles, 
form  harrier  reefs,  such  as  the  keys  off  the  coast  of  Plor- 


222  PHYSICAL  GEOGRAPHY 

ida.  Such  reefs  are  separated  from  the  mainland  by  a 
lagoon  which  is  frequently  deep  enough  for  a  ship  channel. 
By  subsidence  of  the  island  a  fringing  reef  may  be- 
come a  barrier  reef  and  in  time  an  atoll,  or  circular  reef 
inclosing  a  body  of  salt  water  or  lagoon.  Atolls  may  al- 
so be  formed  by  coral  growth  on  the  rim  of  an  extinct 
volcano  or  on  any  sea  bottom  less  than  100  feet  deep. 
Whitsunday,  Caroline  and  many  other  islands  in  the  Pa- 
cific are  atolls.     (See  fig.  162.) 


Pig.  162.  Portion  of  Pinaki  Atoll  and  barrier  reef.  Paumotu  gn^oup, 
Pacific  Ocean.  The  semi- circular  light  band  of  breakers  marks  the 
position  of  the  barrier  reef  surrounding  the  atoll.      (U.   S.  Fish  Com.) 

181.  Fossil  Reefs.— There  are  fossil  coral  reefs  in  the 
limestone  beds  at  Syracuse,  New  York,  at  the  falls  of  the 
Ohio  River  at  Louisville,  Kentucky,  at  Chicago,  Illinois, 
and  many  other  places  in  the  United  States,  signifying 
that  these  areas  were  at  one  time  covered  by  the  sea,  with 
conditions  favorable  to  coral  growth.  What  does  this  in- 
dicate regarding  the  climate  of  central  and  northern  United 
States  in  times  past  ? 


SHORE  LINES 


223 


182.    Limestone  from  Other  Animals  than  Coral.— 

Growing  in  the  same  waters  with  the  corals,  is  a  great 
variety  of  other  animals  and  plants,  many  of  which  secrete 
carbonate  of  lime,  while  others  deposit  silica.     Such  are 


Fig.  1().3.  Shell  beach,  Lagoon  of  Pinaki,  Paumotu  group,  Pacific 
Ocean.  The  beach  is  composed  of  shells  of  mollusks  which  in 
time  will  form  a  bed  of  limestone  similar  to  many  that  now 
occur  on  the   continents.      (U.    S.   Fish   Com.) 


the  different  kinds  of  molluscs,  crinoids  and  sponges. 
There  are  also  many  microscopic  forms.  The  aggregate 
remains  of  the  multitude  of  different  forms  are  mingled 
with  the  corals  in  the  formation  of  the  coral  limestone 
beds.  In  many  places  extensive  beds  of  limestone  or  sili- 
cious  rock  are  formed  by  the  molluscs  and  other  forms  of 
life   without  any  reef -building  coral.      (Fig.    163.)     The 


224  PHYSICAL  GEOGRAPHY 

coquina  limestone  now  forming  in  this  way  along  the  Flor- 
ida coast  is  used  to  some  extent  for  building  stone. 

183.  Lake  Shores.— Lake  shore  lines  are  similar  to 
ocean  shore  lines  in  many  ways.  The  lake  waves  are 
neither  so  large  nor  so  strong  as  the  ocean  waves,  hence 
the  erosion  is  not  so  rapid.  The  water,  except  that  in 
the  salt  lakes,  is  not  so  dense,  hence  sediment  is  not  car- 
ried as  freely  as  in  the  salt  waters.  In  the  northern  lati- 
tudes the  lakes,  except  a  few  of  the  largest  ones,  freeze 
over  in  the  winter  season,  and  are  not  subject  to  active 
erosion  on  the  shore  by  the  winter  winds.  The  ice,  while 
protecting  the  shore  from  the  winds,  becomes  an  active 
agent  of  erosion  in  itself.  The  expansion  of  the  ice  due  to 
changes  in  temperature  causes  it  to  push  against  the  shore 
with  great  force.  When  the  frozen  surface  is  broken  up  by 
warm  weather  the  blocks  are  driven  or.  the  shore  by  storm 
winds. 

There  is  no  coral  in  fresh  water,  and  most  of  the  other  forms 
of  life  common  in  the  ocean  do  not  occur  in  the  lakes,  which 
have  a  fauna  and  flora  of  their  own.  In  the  larger  lakes  the 
living  forms  affect  the  shore  life  very  little;  but  in  many  small 
lakes,  vegetation  accumulates  along  the  shore  and  forms  marshy 
plains  which  in  time  cover  the  whole  lake  basin.  Some  of  the 
small  lakes  are  bordered  by  plains  composed  of  marl,  which  con- 
sists of  the  remains  of  animals  and  plants  that  grew  in  the  lake 
in  sufficient  quantities  to  partly,  sometimes  entirely,  fill  it. 

184.  Fossil  Shore  Lines.— How  may  we  recognize  a 
former  shore  line  after  the  body  of  water  which  caused 
it  has  disappeared?  Many  of  the  features  explained  on 
the  preceding  pages  are  characteristic  of  shores  and  not 
found  elsewhere;  hence  a  recognition  of  these  features 
means  the  recognition  of  a  former  shore  line. 

North  of  Lake  Superior,  at  different  elevations  on  the  hills, 
are  boulder  beaches  similar  to  those  at  the  water's  edge  to-day. 
At  other  points  are  the  wave-built  sand-terraces,  similar  to  those 


SHORE  LINES 


225 


forming  on  the  present  shore.  These  old  beaches  are  at  differ- 
ent elevations  above  the  lake,  some  less  than  one  hundred,  some 
more  than  three  hundred  feet  above  the  water.     (See  fig.  155.) 


Fig.  164.  Chazy  limestone,  Chazy,  N.  Y.  Showing  the  fossil  shells,  that 
are  the  remains  of  animals  buried  in  the  mud  in  the  margin  of  the 
shallow  sea  that  covered  that  area  millions  of  years  ago.      (H.  M.  Brock.) 

Along  the  coast  of  California  in  several  places  are  wave-cut 
terraces  many  feet  above  the  sea,  indicating  a  recent  elevation 
of  the  land  that  carried  the  former  shore  line  high  above  the  sea- 
level.     (See  fig.  148.) 

Besides  the  cobble  beaches  and  sand  terraces,  other  shore 
features  that  may  often  be  recognized  on  fossil  lakes  are  wave- 
cut  cliffs,  terraces,  bars,  spits,  hooks  and  deltas. 

Examples.  Surrounding  Great  Salt  Lake,  in  places 
some  miles  from  the  lake,  is  a  prominent  shore  line,  or 
rather  a  series  of  them  indicating  the  levels  of  a  former 
great  lake  which  has  been  called  Lake  Bonneville.  (See 
lig.  155a.) 
.  Lake  Agassiz  in  central  North  America,  Lake  Passaic 
15 


226  PHYSICAL  GEOGRAPHY 

in  New  Jersey,  and  Lake  Iroquois  in  New  York  are  other 
fossil  lakes  which  have  been  recognized  by  their  shore 
lines.     (See  fig.  86.) 


Fig.  165,  Fossil  boulder  beach  on  north  side  of  Adirondack  Mountains,  Covey 
Hill,  Canada,  480  feet  above  tide.  Similar  boulder  beaches  bordered  by 
shore    cliffs    occur    at    lower    levels.      (H.    M.    Brock.) 

185.  Sand  Dunes.— On  sandy  shores  where  the  pre- 
vailing winds  are  from  the  water,  the  sand  forms  great 
ridges  or  dunes,  varying  in  height  from  a  few  feet  to  sev- 
eral hundred  feet.  In  an  open  country  the  sand  ridges 
are  generally  formed  at  right  angles  to  the  direction  of 
the  winds.  Where  there  are  local  obstructions,  the  small 
dunes  are  parallel  with  the  direction  of  the  winds.  The 
blowing  inland  of  the  sand  from  the  shore  often  aids  ma- 
terially the  cutting  back  of  the  shore  line  by  the  waves. 

On  the  coast  of  southwestern  France  the  prevailing  west 
winds  have  blown  the  sand  inland,  forming  dunes  several  miles 
in  width,  which  have  covered  farms  and  villages  as  they  were 


SHORE  LINES 


227 


driven  forward  by  the  winds.  The  further  progress  of  these 
dunes  has  been  checked  by  planting  trees  over  and  in  front  of 
them,  which  in  some  places  not  only  stop  the  drifting  dunes  but 
even  form  productive  forests.     (Fig.  167.) 


Fig.  166.  iSaiid  duiit-  at  Biggs,  Oregon,  covered  with  wind  ripples.  Wliich 
way  was  the  wind  blowing  that  formed  the  dune  ?  Notice  the  alluvial 
terraces  in  the  background.      (U.  S.  Geol.  Survey.) 


The  eastern  and  southern  shores  of  Lake  Michigan  contain 
many  sand  dunes  of  great  size,  which  have  in  places  made  in- 
roads on  the  fertile  farm  land.  Attempts  have  been  made  to 
check  their  further  progress  by  the  growth  of  grass  and  trees. 
Similar  but  smaller  dunes  occur  in  large  numbers  in  New  York 
State  at  the  east  end  of  Lake  Ontario.  They  are  very  abundant 
in  many  places  along  the  Atlantic  coast,  especially  in  the  Caro- 
linas' 

Sand  dunes  vary  greatly  in  height.  On  the  coast  of  Holland 
some  are  two  hundred  and  sixty  feet;  on  Cat  island,  one  of  the 
Bahamas,  nearly  four  hundred  feet;  and  on  the  western  coast  of 
Africa  near  Cape  Bajador,  more  than  five  hundred  feet  high. 

Sand  dunes  are  also  common  features  on  many  sandy  areas 


228 


PHYSICAL  GEOGRAPHY 


remote  from  and  independent  of  shore  lines.  They  are  very- 
abundant  in  desert  and  semi-arid  areas  and  along  many  river 
flood  plains  in  humid  areas.     (See  fig.  222.) 


Fig.  167.  Diagram  showing  method  of  checking  drifting  sand.  A  solid 
bj^rier,  A,  causes  a  dune  in  front  of  it.  An  open  but  rigid  barrier,  B, 
causes  the  dune  to  form  in  and  finally  over  it.  An  open  flexible  barrier. 
C,  causes  the  dune  to  form  behind.  The  sand  moves  from  left  to-  right 
in  the  diagram.      (After  the  U.   S.   Dept.  of  Agriculture.) 

186.  Harbors.— The  most  important  economic  fea- 
tures along  the  sea  coast  are  the  harbors,  or  places  of 
shelter  for  vessels  in  time  of  storm.     In  some  places  there 


SHORE  LINES 


229 


Fig.   168.     Dune  Park,   Ind.     Advancing  front  of  a  moving  dune,   bury- 
ing forest  and  swamp  vegetation.      (H.  C.  Cowles. ) 


WM 

'.-■^^''^■j^H 

T 

^Ij^^^k 

^n''   ■i'-^MmJ 

j^i^^^mj^^^B 

B|g 

1 

Fig.  169.  Dune  Park,  Ind.  Roots  of  the  cottopwood  tree  exposed  by  the 
action  of  the  wind.  In  time  the  tree  will  be  completely  undermined 
and  overthrown.      (H.  C.  Cowles.) 


230  PHYSICAL  GEOGRAPHY 

are  long  stretches  of  coast  line  without  any  cities,  because 
there  is  no  harbor  for  the  vessels. 

The  conditions  necessary  for  a  good  harbor  are:  (1) 
protection  from  incoming  heavy  waves,  (2)  an  open,  deep 
channel  extending  from  the  anchorage  to  the  open  sea,  (3) 
water  deep  enough  to  permit  the  vessels  to  approach  close 


Fig.   170.     Fiord  Harbor  on  coast  of  Norway.     Many  snch  harbors  lie  well 
inland,  sheltered  from  the  winds  by  high  hills.     Naero  Fiord,  Norway. 

to  the  shore  line  to  facilitate  loading  and  unloading,  (4) 
location  convenient  to  natural  highways  into  the  interior, 
(5)  roomy  enough  to  accommodate  many  vessels  without 
interference,  (6)  good  bottom  for  anchorage,  and  (7) 
absence  of  strong  river  or  tidal  currents. 

(1)  Delta  harbors,  on  the  delta  of  a  great  river,  have  the  ad- 
vantage of  access  by  water  to  the  great  fertile  plains  of  the  in- 
terior of  the  continent,  but  they  are  often  hampered  by  the 
diflaculty  of  keeping  a  ship  channel  open  and  free  from  the 
mass  of  the  mud  carried  in  by  the  river. 


SHORE  LINES  231 

(2)  Estuary  delta  harbors  are  on  drowned  rivers  where  the 
sea  has  entered  the  lower  part  of  the  valley  and  a  new  delta  has 
formed  at  the  head  of  the  bay  or  estuary. 

(3)  Fiord  harbors  differ  from  the  preceding  in  being  deeper, 
and  generally  lying  in  rock  depressions  with  less  soil  on  the 
bordering  hills  than  commonly  occurs  along  the  bays  or  estua- 
ries. The  fiords  represent  the  deep  ice-worn  channels  of  glacial 
origin,  and  hence  are  found  only  in  high  latitudes  where  the 
glacial  streams  run  into  the  sea.  Their  origin  accounts  for  the 
bare  rock  walls  and  scarcity  of  mantle  rock.  They  are  abundant 
on  the  coast  of  Norway.     (Fig.  170.) 

(4)  Mountain  ranges  that  project  into  the  sea  frequently 
have  troughs  or  depressions  below  sea  level  which  may  be  util- 
ized as  harbors.  Such  is  the  western  end  of  the  Pyrenees  in 
Spain,  and  the  peninsula  and  islands  of  Greece.  Sometimes  the 
mountains  are  parallel  to  the  coast,  and  the  harbor  or  harbors 
may  lie  inside  the  first  range,  as  San  Francisco  Bay  and  many 
similar  land-locked  areas  along  the  coast  of  Washington  and 
Alaska. 

(5)  Glacial  moraine  deposits  along  a  sea  coast  sometimes 
form  protected  harbors. 

(6)  Lagoon  and  sand-bar  harbors  occur  on  almost  all  sandy 
shores  where  there  is  a  long  stretch  of  shallow  sea  bordering 
the  coast.  The  waves  build  up  a  barrier  at  some  distance  off 
shore,  and  the  lagoon  between  the  bar  and  the  shore,  where  deep 
enough,  furnishes  safe  anchorage  for  vessels.  Most  of  the  la- 
goons of  this  kind  are  not  deep  enough  for  the  modern  ocean 
steamship,  unless  deepened  artificially  by  dredging,  but  they 
serve  a  useful  purpose  for  the  smaller  coasting  vessels. 

(7)  Sand-spit  harbors  are  similar  in  some  respects  to  the  last 
mentioned,  but  in  the  spit  the  sand  is  drifted  along  the  shore 
until  in  drifting  past  the  headland  at  the  entrance  of  a  bay  the 
spit  is  carried  part  way  or  perhaps  entirely  across  the  bay,  thus 
making  a  safe  anchorage  in  the  bay  behind  it.  Provincetown,  on 
Cape  Cod  is  an  example  of  this  class. 

(8)  Volcanic  crater  harbors  are  formed  by  a  breach  in  the 
rim  of  a  volcanic  crater  that  occurs  near  sea  level  on  an  island 
or  the  border  of  a  continent.  The  accompanying  view  (fig.  171) 
shows  such  a  crater  at  the  village  of  Ischia,  on  the  island  of 
Ischia,  near  Naples.     The  notch  in  the  rim  of  the  crater  permits 


232 


PHYSICAL  GEOGRAPHY 


small  vessels  to  enter  and  find  a  snug  harbor  in  the  crater  of 
the  extinct  volcano. 

(7)  Coral  reefs  and  atolls  furnish  many  much-needed  harbors 
in  the  tropics.  The  lagoon  inside  of  the  barrier  reef  or  on  the 
interior  of  an  atoll  frequently  furnishes  a  good  harbor  for  large 
as  well  as  small  vessels.  Often  the  entrance  to  the  coral  har- 
bor is  narrow,  intricate  and  dangerous.  Biscayne  bay,  on  the 
east  coast  of  Florida,  is  an  example  of  a  coral  barrier-reef  har- 
bor; and  Hamilton,  on  the  Bermudas,  is  an  example  of  an  atoll 
harbor.  Both  of  these  types  are  much  more  numerous  in  the 
Pacific  than  in  the  Atlantic  ocean. 


Fig.  171.  A  volcanic  crater  harbor  on  the  Island  of  Ischia  in  the  Bay 
of  Naples,  Italy.  A  notch  in  the  rim  of  the  crater  forms  an  opening 
through  which  boats  pass  to  the  open   sea. 

187.  Economic  Importance  of  Haxbors.— The  pres- 
ence or  absence  of  good  harbors  has  much  to  do  with  the 
location  of  cities  and  the  commercial  prosperity  of  the 
adjacent  region.  The  location  of  San  Francisco  is  not  an 
accident.     It  has  one  of  the  best  harbors  on  the  Pacific 


SHORE  LINES  233 

coast,  which  is  at  the  same  time  a  connecting  link  between 
the  ocean  and  the  great  fertile  interior  valley  of  Califor- 
nia. New  York  City  on  the  east  coast  is  the  metropolis  of 
the  United  States  mainly  because  of  its  good  harbor,  lo- 
cated at  the  natural  doorway  into  the  interior  of  the  con- 
tinent through  the  Hudson  Valley  and  the  Great  Lakes. 
Boston,  Philadelphia,  Baltimore,  Washington,  and  all  the 
prominent  cities  on  the  eastern  coast  owe  their  locations 
mainly  to  their  good  harbors. 

In  a  few  places  there  are  sufficient  attractions  to  cause 
the  growth  of  a  town  or  city  where  there  is  no  harbor,  and 
the  commerce  is  carried  on  under  great  difficulties.  Nome 
City  on  Cape  Nome,  Alaska,  is  an  example.  Great  quan- 
tities of  gold  are  found  in  the  sands  and  gravels  of  the 
sea  shore  and  the  inflowing  streams.  People  come  to  get 
the  gold  and  must  have  food,  clothing,  houses,  machinery, 
etc.,  which  are  brought,  by  ships  on  the  ocean,  but  they 
cannot  get  close  to  the  shore  because  of  the  shallow  water. 
The  goods  and  passengers  are  unloaded  by  lightering,  that 
is,  a  lighter  vessel  or  boat  takes  a  portion  of  the  freight 
through  the  breakers  and  shallow  water  to  the  shore. 
Sometimes  horses  and  cattle  are  thrown  into  the  water  and 
made  to  swim  ashore.  In  time  of  a  storm  the  lighter  can- 
not get  through  the  breakers,  and  the  large  vessels  must 
wait  until  the  storm  subsides,  which  may  be  several  days. 
In  a  violent  storm  it  must  put  out  to  sea  to  avoid  being 
driven  aground  in  the  shallow  water  and  destroyed  by 
the  waves. 

In  the  absence  of  a  natural  harbor,  sometimes  an  arti- 
ficial harbor  is  constructed  at  a  great  expense.  This  is 
done  by  building  a  wall  or  breakwater  out  from  the  shore, 
inclosing  water  deep  enough  to  float  the  vessels.  The 
breakwater  is  so-called  because  on  it  the  heavy  storm- 
waves  from  the  open  sea  break  and  lose  their  power  to 


234  PHYSICAL  GEOGRAPHY 

injure  a  vessel  lying  safely  in  the  calm  waters  behind  the 
wall.  The  harbor  for  the  city  of  Los  Angeles  at  San 
Pedro  on  the  Pacific  Coast  is  a  type  of  this  class.  See  the 
Oswego  sheet  of  the  U.  S.  Topographic  Atlas  for  example 
of  artificial  harbor  at  Oswego. 


REFERENCES 

Shaler,  Beaches  and  Tidal  Marshes,  Natl.  Geog.  Mon.,  Amer. 

Book    Co.    Natural    History    of    Harbors,    13th    An. 

Rept.  U.  S.  Geol.  Survey. 
Gulliver,  Shore  Line  Topography,  Proc.  Am.  Acad.  Arts  and 

Sci.,  Vol.   34,  Jan.,   1899. 
Gilbert,  Features  of  Lake  Shores,  5th  An.  Rept.  U.  S.  Geol. 

Surv.  p.  75. 
Darwin,  Structure  and  Distribution  of  Coral  Reefs,  Appleton 

&  Co.,  New  York,  1889. 
Dana,    Corals    and   Coral   Islands,   Dodd,    Mead    &    Co.,    New 

York,  1895. 


CHAPTER  VII 

THE  LAND 

The  land  or  solid  portion  of  the  earth,  has  many  fea- 
tures in  strong  contrast  with  the  water,  or  liquid  portion, 
and  the  atmosphere,  or  gaseous  portion.  It  is  subject  to 
change  like  the  other  parts.  The  mountains,  the  plains, 
and  even  the  rocks  themselves  undergo  cycles  of  change, 
each  with  its  own  life-history,  extending  over  a  very  long 
period  of  time. 

So  slow  are  the  changes  that  to  the  casual  observer  the 
mountains  were  always  mountains  and  will  ever  remain 
such,  but  the  geographer  sees  the  mountains  and  hills  in 
process  of  growth  and  decay,  and  to  him  they  are  living 
objects  of  interest  as  he  studies  their  varied  changes.  So 
with  the  plains,  plateaus,  valleys,  volcanoes  and  other 
natural  features.  He  studies  carefully  the  variations 
going  on  and  learns  that  by  properly  interpreting  these 
he  can  trace  out  the  birth,  growth,  maturity,  decay  and 
disappearance  of  even  the  ''everlasting  hills."  Their 
previous  history  is  recorded  and  preserved  in  the  rock 
strata  which  have  been  wisely  compared  to  the  stone 
leaves  in  the  book  of  nature  from  which  the  geologist  reads 
the  history  of  the  earth  and  its  development.  The  study 
of  this  history  is  properly  the  province  of  Geology.  It  is 
the  province  of  Geography  to  interpret  the  elements  of 
this  history  and  study  the  ways  in  which  it  is  made  and 
recorded. 

188.  Divisions.— The  two  main  continents  have  been 
named  the  Eastern,  comprising  Eurasia  and  Africa,  and 
the    Western,    comprising    North    and    SoutH    America. 

235 


236  PHYSICAL  GEOGRAPHY 

Australia  is  a  tliird  continent  much  smaller  than  either  of 
the  other  two;  and  probably  Antarctica,  of  which  little  is 
known,  may  prove  to  be  a  fourth.  Continents  are  dis- 
tinguished from  islands;  first,  by  size,  being  much  larger; 
and  second,  by  structure,  since,  except  the  coral  atolls, 
the  interior  of  islands  is  high  land  sloping  to  the  sea,  while 
the  interior  of  the  continents  is  lowland  consisting  of 
great  river  basins.  The  high  land  and  great  mountain 
ranges  occur  mostly  on  or  near  the  margin  of  the  continent, 
and  not  in  the  interior,  as  in  the  case  of  islands. 

189.  Islands. — Islands  may  be  divided  into  two 
classes,  continental  and  oceanic;  the  first  includes  those 
that  lie  near  the  continents  on  the  continental  shelf  and 
hence  are  surrounded  by  comparatively  shallow  water; 
the  second,  those  that  rise  out  of  the  deep  waters  of  the 
ocean  basins. 

Continental  islands  are  of  two  kinds;  (1)  those  built 
up  on  the  shallow  ocean  bottom  by  corals  or  by  currents, 
waves,  and  wind,  such  as  the  sand  barriers  along  the 
Atlantic  coast;  (2)  Remnants  of  the  continent  left  above 
the  sea  level  by  an  advancing  shore  line.  The  hills  of  the 
old  shore  become  islands  along  the  new  shore.  Such  are 
the  islands  along  the  coast  of  Maine.  Those  formed  in 
the  first  way  are  low  and  sandy  with  sandy  shores.  Those 
in  the  second  are  generally  rocky  and  bordered  by  rock 
cliffs. 

190.  Distribution  of  the  Land.— Tt  may  be  observed  on  a 
globe  or  a  map  of  the  continents  that  much  more  than  half  of 
the  land  area  lies  north  of  the  equator.  If  we  should  divide  the 
globe  into  two  hemispheres  by  taking  London,  the  greatest  cfty 
in  the  world,  as  a  center  of  one  of  them,  it  would  be  seen  that 
nearly  all  of  the  land  (about  nine-tenths)  is  included  in  the  Lon- 
don Hemisphere,  while  the  other  is  largely  a  water-area.  Is 
there  any  significance  in  the  location  of  London  in  the  center 
of  the  land  hemisphere? 


THE  LAND 


237 


Since  it  is  the  land  masses  that  divide  the  ocean  into  its 
different  parts,  the  unequal  distribution  of  the  land  causes  an 
unequal  division  of  the  oceans;  thus  the  Pacific  ocean  which  lies 
mainly  in  the  water-hemisphere  is  much  larger  than  any  of  the 
other   oc"eanic   divisions. 

191.  Importance  of  the  Land.— To  the  existence  of 
dry  land  is  due  the  possibility  of  all  forms  of  land  life. 


Water  Hemlspbdre.  hand  Hemlspliere. 

Fig.  172.  Division  of  the  earth  into  two  hemispheres,  one  of  which  contains 
nearly  all  the  land.  London  is  near  the  center  of  the  land  hemisphere. 
Where   is  tHe   center  of  the  water  hemisphere  ? 

It  is  possible  that  a  large  part  of  the  life  in  the  sea  is 
dependent  on  the  occurrence  of  land,  since  much  of  the 
material  to  supply  the  marine  life  comes  from  the  land. 
To  the  land  areas  are  due  likewise  the  direction,  intensity, 
and  in  large  measure  the  existence  of  the  ocean  currents 
so  important  to  life  both  in  the  sea  and  on  the  land. 
They  also  influence  in  large  measure  the  movements  of  the 
atmosphere  and  the  distribution  of  moisture  in  the  form  of 
rain  and  snow.  Imagine  what  a  monotonous  and  dreary 
planet  this  earth  would  be  if  the  entire  surface  were  cov- 
ered with  water  as  it  would  be  if  the  rock  surface  were  free 
from  elevations  and  depressions. 


238 


PHYSICAL  GEOGRAPHY 


192.  Topography  of  the  Land  Compared  With  the 
Sea  Bottom. —  In  its  broad,  general  features,  the  land 
areas  somewhat  resemble  the  ocean  floor  in  reverse  order. 
The  continents  correspond  in  a  way,  above  the  ocean  level, 
to  the  ocean  basins  below  except  that  they  are  smaller. 
The  great  plateaus  on  land  have  their  opposites  in  the  anti- 
plateaus  or  deeps  of  the  ocean.  Here  the  analogy  ceases, 
as  there  is  no  erosion  over  the  ocean  bottom  to  correspond 
to  the  multitude  of  valleys  and  steep  hillsides  on  the  land. 
The  widespread  monotony  of  the  ooze-covered  plains  of 
the  ocean  bottom  is  replaced  on  the  land  by  an  ever  vary- 
ing diversity  of  landscape  produced  by  the  carving  action 
of  the  rainfall  and  the  streams  on  the  uplifted  plateau  and 
mountain  masses. 


\^a..-s-^^"^'^ 


Potassium  1 .  Z" 
All  others  not    \  ./<>■ 


Fig.  173.  Diagram  showing  the  approximate  percentage  of  the  common  ele- 
ments that  form  the  known  part  of  the  earth.  More  than  half  of  the 
oxygen  is  combined  with  the  silicon.     (Hessler  and  Smith.) 

193.    The  Composition  of  the  Earth's  Crust.—     The 

solid  portion  of  the  earth  contains  a  greater  variety  of 
chemical  elements  than  the  atmosphere  or  the  hydro- 
sphere. About  eighty  different  elements  have  been  recog- 
nized by  the  chemists,  but  only  a  few  of  these  form  an 


THE  LAND  239 

appreciable  part  of  the  rocks  and  the  minerals  of  the 
earth's  crust.  One  element,  oxygen,  forms  about  half  of 
all  the  known  part  of  the  earth,  including  the  air,  water 
and  land.  The  other  most  common  elements  are  silicon, 
aluminium,  calcium,  magnesium,  iron,  sodium,  potassium, 
carbon,  hydrogen,  and  nitrogen.  The  elements  enter  into 
many  different  mineral  and  rock  combinations,  but  again 
the  bulk  of  all  the  rocks  is  made  up  of  a  comparatively 
small  part  of  the  hundreds  of  known  minerals.*  (See  fig. 
173.) 

194.  Minerals. —  A  mineral  is  a  portion  of  inorganic 
homogeneous  material  produced  by  natural  means,  having 
a  definite  or  nearly  definite  chemical  composition  and  gen- 
erally having  a  crystalline  structure.  Most  minerals  are 
crystalline  and  many  have  a  definite  crystal  form,  but 
many  occur  in  rock  masses  where  the  crystal  form  does  not 
appear,  while  some  few  have  not  even  crystalline  texture. 

Minerals  are  distinguished  from  each  other  by  a  care- 
ful comparison  of  all  the  physical  properties  such  as  hard- 
ness, color,  color  of  the  powder  or  streak,  crystal  form  and 
habit,  cleavage,  luster,  optical,  electrical,  and  magnetic 
properties,  all  of  which  can  be  learned  satisfactorily  only 
by  the  study  of  the  mineral  specimens.  They  may  also  be 
distinguished  by  their  chemical  properties  which  may  be 
tested  by  the  use  of  the  blowpipe  with  different  reagents. 
Minerals  are  classified  commercially  according  to  their 
uses,  and  scientifically  according  to  their  composition. 

Suggestions:  In  studying  the  hardness  of  minerals  it  is  cus- 
tomary to  select  a  certain  number,  generally  ten,  and  arrange 
them   in  the    order   of   relative   hardness,    as   a   scale   for   com- 

*It  is  not  necessary  to  study  all  the  minerals  described  in  the  following 
pages.  The  student  should  study  the  minerals  in  his  laboratory  collection 
and  use  these  pages  for  reference.  Do  not  study  the  text  on  minerals  with- 
out the   minerals,   but   in   connection  with  them. 


240  PHYSICAL  GEOGRAPHY 

parison.  The  ones  commonly  selected  are  (1)  talc,  (2)  gypsum, 
(3)  calcite,  (4)  fiuorite,  (5)  apatite,  (6)  orthoclase,  (7)  quartz, 
(8)  topaz,  (9)  corundum,  (10)  diamond.  By  comparing  any  other 
mineral  with  these  its  hardness  can  be  designated  by  a  number 
corresponding  to  that  given  ti  the  mineral  with  which  it  agrees 
in  the  scale.  For  example,  a  mineral  that  would  scratch  calcite, 
but  not  fluorite  would  be  marked  4  in  the  scale  commonly  writ- 
ten H  (=)  4.  The  powder  or  streak  may  be  obtained  by  rub- 
bing the  mineral  on  unglazed  porcelain  or  by  scratching  it  with 
a  file. 

All  crystal  forms  of  materials  may  be  divided  *into  six  groups 
or  systems,  but  the  determination  of  these  involves  more  knowl- 
edge of  crystallography  than  can  be  given  here.  Compare  the 
crystals  as  to  the  number  of  faces,  the  number  of  the  same  kind, 
the  shape  of  the  faces  and  the  arrangement  of  them.  The  dif- 
ferent kinds  and  degrees  of  cleavage  and  luster  can  be  learned 
by  comparison  of  the  different  minerals. 

Rock-forming  minerals.  The  most  important  rock- 
forming  minerals  are  quartz,  the  feldspars,  micas,  horn- 
blende, augite,  calcite,  dolomite,  serpentine  and  kaolin. 

195.  Quartz,  the  oxide  of  silicon  (SiO^ ),  forms  one  of 
the  essential  minerals  in  granite,  the  bulk  of  the  grains  in 
most  of  the  sandstones,  and  a  part  of  some  other  rocks.  It 
is  one  of  the  hardest  of  the  common  minerals,  7  in  the 
scale,  readily  scratching  glass.  It  crystallizes  in  six-sided 
prisms  with  pyramids  (See  fig.  174).  It  has  a  conchoidal 
fracture,  and  commonly  a  vitreous  luster,  some  varieties 
have '  a  waxy  luster  and  others  a  dull  luster.  While  it 
occurs  in  nearly  all  the  different  colors,  in  the  granites 
and  sandstones  it  Is  usually  gray,  white,  or  colorless.  The 
streak,  difficult  to  obtain,  is  white  or  gray.  In  the  mineral 
form  it  is  used  commercially  in  the  manufacture  of  glass 
and  porcelain.  Some  varieties,  as  rock  crystal,  amethyst, 
jasper,  and  chrysoprase  are  used  as  gems.  Some  sand- 
paper is  made  from  ground  quartz.  Compare  quartz  with 
calcite,  feldspar,  fluorite,  and  selenite. 


/  Cute. 


^' 


/d  fyritohejtoti. 
Rrite 


5   OctotieJron. 
Mametite^  ffhte, 
Fluotite. 


Heyaaona/ 
4-.   fPismanc/lframicl.     S^Qa^rfz  Crystal. 
Quartz. 


n\ 


6.  Ocwenohec/tcn. 
CJcite-DojfootIf  Sia^r. 


tiJ 


7  Rhombohec/ron. 
Cleavage  form  ofCalcitc. 


0-  nflonoclinic.  y.  Irafsezohetfton. 

Orthoclase  feUspar.        Gdrhet. 

Fig.  174.  Some  of  the  common  crystal  forms.  The  same  mineral  may  occur 
in  different  forms,  as  pyrite  in  the  first  three  above,  but  for  the  same 
mineral  these  are  always  in  the  same  one  of  the  six  systems.  The  three 
forms  given  for  pyrite  are  in  the  isometric  systems.  The  two  forms  for 
quartz,  Nos.  4  and  5  are  in  the  hexagonal  svstem. 
16 


242  PHYSICAL  GEOGRAPHY 

196.  Feldspars  are  divided  into  two  classes,  ortho- 
clase_  and  plagioclase.  The  first  occurs  in  granites  and 
syenites,  the  other  in  the  diorites  and  gabbros.  The  ortho- 
clase  contains  potash  combined  with  silica  and  alumina, 


Fig.  175.  Half  of  a  large  feldspar  (al])ite)  crystal  from  Cabot,  Vt.,  4  feet 
long  2y2  feet  wide.  The  feldspai-s  and  quartz  sometimes  form  very 
large  crystals.      (C.  H,  Richardson,  Vt.  Geol.   Survey.) 

while  plagioclase  contains  soda  or  lime  or  both  in  place  of 
the  potash.  The  feldspars  are  not  quite  so  hard  as  quartz 
being  one  less  in  the  scale,  but  are  still  hard  enough  to 
scratch  glass.  They  differ  from  quartz  in  having  bright 
cleavage  surfaces  in  two  directions  at  right  angles  or 
nearly  so.  Feldspar  becomes  dull  on  weathering  as  it 
disintegrates  and  finally  crumbles  into  a  soft,  clayey  mass. 
Feldspar  is  commonly  white,  gray,  or  pink  in  color.  It 
is  quarried  in  New  York,  Pennsylvania,  New  Hampshire, 
and  elsewhere  and  is  used  in  the  manufacture  of  porcelain 
and  chinaware.     (See  fig.  175). 


THE  LAND  243 

197.  Micas  are  characterized  by  the  extremely  thin 
plates  or  scales  into  which  they  may  be  separated,  due  to 
the  perfect  cleavage.  There  are  several  different  varieties, 
of   which   muscovite   and   biotite   are   the   most    common. 


VlQ.  17C.  Feldspar  quarry  near  Elam,  Pa.,  August,  1898.  Feldspar 
occurs  in  commercial  form  in  veins  or  dikes  in  igneous  or  metamorphic 
rocks.     It  forms  a  large  part  of  the   granite  rocks. 

Muscovite,  the  so-called  isinglass,  is  colorless  in  thin  pieces 
when  pure.  It  is  used  for  electrical  purposes,  lanterns, 
stove  and  furnace  doors,  as  a  lubricant  and  for  decorative 
purposes.  It  occurs  in  granite,  syenite,  and  in  some 
schists,  and  sandstones.  Biotite  is  black  or  dark  green  in 
color  and  occurs  in  granite,  syenite,  and  some  schists,  also 
in  diorite  and  some  of  the  other  dark-colored  igneous  rocks. 
Muscovite  has  a  composition  somewbat  similar  to  that  of 


244  PHYSICAL  GEOGRAPHY 

orthoclase ;  in  biotite,  iron  and  magnesia  replace  the  potash 
of  the  muscovite. 

198.  There  are  several  varieties  x)t  amphihole,  the  most  com- 
mon of  which  is  hornhlende,  a  black  mineral  occurring  in 
syenite,  diorite,  some  granites  and  schists.  It  may  be  dis- 
tinguished from  biotite  by  not  separating  in  thin  scales.  One 
form  of  the  fibrous  mineral,  asbestos,  is  a  variety  of  hornblende. 
Another  form  of  asbestos  is  a  variety  of  serpentine.  It  is  the 
latter  that  is  used  extensively  for  making  fire-proof  cloth,  and 
covering  steam  pipes  and  boilers. 

199.  Augite,  (the  most  common  of  the  pyroxenes)  is  a  dark 
green  mineral  which  occurs  in  diabase,  basalt,  and  gabbro. 
Augite  and  hornblende  are  important  rock-making  minerals  com- 
posed of  silicates  of  iron,  magnesia,  and  lime.  Varieties  of  each 
differ  in  color  but  where  they  form  a  large  part  of  the  rock  mass, 
the  augite  is  dark  green  and  the  hornblende  is  black.  The 
augite  is  commonly  in  short,  thick  crystals  or  irregular  masses, 
hornblende  usually  in  long,  slender  crystals,  sometimes  finely 
fibrous. 

200.  Calcite  is  composed  of  the  carbonate  of  calcium 
(CaCO^)  and  forms  the  bulk  of  the  limestones,  marbles, 
and  chalk  deposits.  It  forms  a  large  part  of  marl,  of 
shells  of  all  kinds,  most  of  the  coral,  extensive  deposits  in 
caves  and  about  lime  springs  and  occurs  in  veins  or  fissures 
in  different  kinds  of  rocks.  The  limestones  and  marbles 
besides  having  extensive  use  as  building  and  ornamental 
stone  are  used  for  making  quicklime  and  cement  and  hence 
they  form  the  base  of  most  of  the  mortars  in  building 
operations.  Calcite  is  one  of  the  most  useful  of  all  the 
minerals  and  fortunately  is  very  widely  distributed. 
Compare  calcite  with  dolomite,  quartz,  and  fluorite  and 
point  out  the  differences,  telling  how  you  would  distin- 
guish them.  When  pure  it  is  colorless  to  white;  impure 
varieties  occur  in  all  colors— red,  black,  blue,  gray,  and 
yellow  are  abundant.  It  cleaves  readily  in  three  direc- 
tions  into    rhombohedrons.     Clear    forms,    Iceland    spar, 


THE  LAND 


245 


show    double   refraction.     It    effervesces    freely   in    dilute 
acid.     (See  figs.  177  and  174.) 

201.     Dolomite    is  the  double  carbonate  of  lime  and 
magnesia,  and  hence  differs  from  calcite  in  having  part 


Fig.  177.  Travertine  quarry  at  Bagn^,  near  Rome,  Itaiy.  The 
rock  is  calcite  deposited  from  solution  in  the  spring  water. 
From  this  quarry  the  rock  was  obtained  for  the  Coliseum, 
St.  Peters,  and  other  buildings  in  Rome.      (J.  C.  Branner.) 

of  the  lime  replaced  by  magnesia.  It  frequently  resembles 
calcite  so  closely,  especially  in  many  limestones  and 
marbles,  that  it  is  difficult  to  distinguish  from  it.  It  may 
commonly  be  distinguished  by  adding  quite  dilute  cold 
hydrochloric  acid,  in  which  the  calcite  will  effervesce 
vigorously  and  the  dolomite  but  little,  if  at  all,  until  the 
acid  is  heated. 

202.  Kaolin  when  pure  is  white  and  forms  China 
clay.  It  is  formed  by  the  decomposition  of  the  feldspars 
and  the  other  silicates  by  the  action  of  the  groundwater 
leaching  out  the  metallic  bases,  leaving  the  insoluble  sili- 
cate of  alumina,  which  combined  with  water  forms  kaolin. 
It  is  used  in  the  manufacture  of  china  and  porcelain  ware. 


246  PHYSICAL  GEOGRAPHY 

encaustic  tile,  and  as  a  filler  for  paper.  Mixed  with  other 
materials  it  probably  forms  the  bulk  of  all  the  clays  and 
shales,  and  a  considerable  portion  of  all  the  soils. 

ORES 

The  ores  are  the  minerals  from  which  the  metals  are 
obtained.  A  few  of  the  metals  such  as  gold  and  copper 
occur  in  the  metallic  state  in  nature  and  are  called  native 
gold,  native  copper;  but  most  commonly  the  metals  in 
nature  occur  combined  Avith  one  or  more  other  elements, 
forming  compounds  known  as  ores.  The  most  common 
combinations  are  with  oxygen,  forming  oxides;  sulphur, 
forming  sulphides;  and  carbonic  acid,  forming  carbonates. 

203.  Iron,  the  most  useful  of  all  the  metals,  occurs  in 
nature  in  all  three  of  the  above  compounds  in  its  different 
ores. 

Hematite  (red  hematite,  fossil  ore,  specular  ore)  is  at 
present  the  most  important  ore  of  iron  in  the  United  States 
and  from  it  more  than  four-fifths  of  our  iron  is  manu- 
factured. It  occurs  in  several  varieties,  some  of  a  bright 
red  color,  some  steel  gray,  and  some  almost  black.  What- 
ever the  color  of  the  mass,  the  streak  or  powder  is  always 
red.  Hematite  consists  of  ferric  oxide,  the  higher  oxide 
of  iron.  (Fe^O  ).  The  most  productive  locality  for  this 
ore  is  the  region  about  Lake  Superior  from  which  much 
of  the  ore  is  shipped  by  boats  on  the  Great  Lakes.  It  is 
mined  extensively  in  Alabama  and  in  smaller  quantities 
in  New  York,  Tennessee,  Virginia,  Missouri,  and  other 
states. 

Limonite  (brown  hematite,  bog  ore,  yellow  ochre)  the 
hydrous  ferric  oxide  (Fe^O^,  2H2O)  differs  in  composition 
from  the  hematite  by  having  water  combined  with 
the  iron  oxide.     It  has  the  same  composition  as  the  iron 


THE  LAND 


247 


Fig.  178.  Map  of  Lake  Superior  iron  ore  district  and  markets.  More  than  80 
per  cent,  of  the  iron  ore  mined  in  the  United  States  comes  from  the  Lake 
Superior  mines.  Much  of  it  is  shipped  by  boat  over  the  Great  Lakes. 
The  district  second  in  importance  is  in  Alabama.  The  shaded  areas  are 
the  coal  fields.  Cities  underlined  are  the  principal  shipping  and  receiving 
points.     Ore  districts  at  Lake  Superior  and  in  Alabama  in  black. 


248  PHYSICAL  GEOGRAPHY 

rust  that  forms  on  iron  objects  exposed  to  the  air.  It 
varies  in  color  from  yellow  ochre  to  very  dark  brown,  al- 
most black,  but  the  streak  is  always  brown  or  yellow.  It 
forms  the  yellow  and  brown  coloring  matter  in  nearly  all 
the  soils  and  mantle  rock.  It  is  deposited  in  bogs  form- 
ing the  bog  ore,  and  occurs  in  many  places  in  the  mantle- 
rock,  especially  in  that  resulting  from  decayed  limestone. 
It  has  been  mined  in  hundreds  of  places  along  the  lime- 
stone areas  in  the  Great  Valley  of  the  Appalachians  and 
elsewhere. 

Magnetite,  the  third  oxide  of  iron,  consists  of  the  union 
of  the  ferric  and  ferrous,  or  the  higher  and  the  lower  iron 
oxides,  (Fe^O  ,  FeO)  and  contains  a  higher  percentage 
of  iron  when  pure  than  any  of  the  other  ores.  It  differs 
from  the  other  oxides  and  from  all  other  minerals  by  its 
strong  magnetic  properties.  Three  other  minerals,  one 
variety  of  hematite  and  the  bronze-colored  iron  sulphide, 
pyrrhotite,  and  franklinite  are  slightly  magnetic,  but  no 
other  minerals  are  attracted  as  strongly  by  a  magnet  as 
magnetite.  Magnetite  is  black  in  color  and  the  streak  is 
black  which  distinguishes  it  from  the  other  iron  ores.  It 
occurs  in  the  Adirondack  Mountains,  in  southeastern  New 
York,  along  the  east  side  of  the  Appalachians  and  else- 
where. One  of  the  largest  magnetite  mines  in  the  world 
is  at  Cornwall  near  Lebanon,  Pennsylvania. 

Iron  pyrites  is  a  yellow,  brass-colored  mineral,  the  sul- 
phide of  iron,  (FeS^)  sometimes  called  ''fool's  gold,*'  be- 
cause so  frequently  mistaken  for  that  precious  metal.  The 
name  is  appropriate  because  despite  the  resemblance  in 
color  it  may  be  so  easily  and  surely  distinguished  from 
gold.  When  placed  in  the  fire  or  on  a  hot  stove,  it  turns 
black,  gives  off  the  odor  of  burning  sulphur  and  becomes 
magnetic.     It  is  hard  and  brittle  while  gold  is  soft  and 


THE  LAND  249 

malleable.  While  commonly  classed  with  the  iron  ores, 
pyrites  are  not  used  for  the  manufacture  of  iron  in  the 
United  States  because  the  sulphur  would  injure  the  quality 
of  the  product.  It  is  used  for  the  sulphur  in  the  manu- 
facture of  paper,  phosphates,  etc.     (See  fig.  174.) 

Siderite,  the  carbonate  of  iron,  (FeCOs)  is  formed  by  the 
combination  of  carbonic  acid  with  the  oxide  of  iron.  It  varies 
in  color  from  gray  to  brown  and  sometimes  blacl^  as  in  the  black- 
band  ore.  It  occurs  associated  with  coal  beds  and  as  black 
nodular  masses  in  the  shale  beds,  where  it  forms  the  clay  iron- 
stone. It  is  used  extensively  in  England  and  formerly  in  this 
country  for  the  production  of  iron,  but  it  is  used  very  little  in 
the  United  States  at  present. 

204.  Copper  Ores.— In  the  Lake  Superior  district 
copper  occurs  in  the  metallic  state,  native  copper,  but  in 
the  western  areas  it  occurs  mostly  in  the  compounds  of 
the  metal  with  carbonic  acid  or  with  sulphur. 

CJialcopyrite,  the  most  common  copper  sulphide,  is  yel- 
low in  color  and.  is  frequently  mistaken  for  iron  pyrite. 
It  differs  from  pyrite  in  being  softer,  hence  more  easily 
scratched,  having  a  more  golden  yellow  color  and  giving 
a  blackish  green  color  in  the  powder.  Bornite,  another 
sulphide,  varies  in  color,  being  blue,  purple,  and  yellow. 
There  are  two  carbonates  of  copper— maZac/ii^e,  having  a 
bright  green  color  and  azurite,  a  deep  blue,  both  of  which 
beside  their  use  as  a  source  of  copper  are  sometimes  used 
for  ornamental  purposes. 

205.  Lead  Ores.— The  most  common  ore  of  lead  is  galena,  a 
sulphide  of  lead  (PbS)  which  looks  much  like  the  metal.  It  crystal- 
lizes in  cubes,  and  has  a  cubical  cleavage,  that  is,  when  broken  it 
parts  along  planes  parallel  to  the  faces  of  the  cube.  Its  cleavage 
combined  with  its  brittleness  distinguish  it  readily  from  the 
metallic  lead  which  it  resembles  in  color.  Its  color,  cleavage, 
and  specific  gravity  (6-7)  distinguishes  it  from  other  minerals. 
Galena  frequently  contains  silver,  and  most  of  the  silver  mines 


250  PHYSICAL  GEOGRAPHY 

produce  large  quantities  of  lead  as  a  by-product.     Some  lead  is 
obtained  from  cerussite,  the  carbonate  of  lead. 

206.  Zinc  Ores.— The  chief  zinc  ore  is  sphalerite  or  zinc 
blende,  a  sulphide  of  zinc,  (ZnS)  called  "jack"  by  the  miners.  It 
has  usually  a  brown  color,  nearly  black  at  times,  and  a  resinous 
luster.  The  most  productive  localities  are  Missouri,  Kansas, 
and  Wisconsin.  In  New  Jersey  much  zinc  is  obtained  from 
franklinite,  a  bluish-black  mineral  resembling  magnetite,  and 
from  zincite,  a  red-colored  oxide  of  zinc.  Willemite,  a  silicate  of 
zinc,  occurs  with  the  New  Jersey  ores. 

207.  Aluminium  Ores.— Bauxite,  mined  in  Alabama  and  Ar- 
kansas, is  practically  the  only  ore  of  aluminium  at  the  present 
time,  although  the  metal  occurs  abundantly  in  many  other  min- 
erals. Cryolite,  formerly  used  almost  entirely  as  a  source  of 
aluminium,  was  at  one  time  "shipped  in  in  large  quantities  from 
Greenland.  Corundum  is  nearly  pure  alumina,  that  is,  the  oxide 
of  the  metal;  emery,  ruby,  and  sapphire  are  varieties  of  corun- 
dum. Aluminium  forms  a  part  of  the  clay  in  all  the  great  clay 
and  shale  beds,  but  it  is  too  difficult  to  separate  from  the  silica 
in  the  clay  to  make  the  clay  a  source  of  the  metal. 

208.  Amongst  other  useful  minerals  are  halite,  gyp- 
sum, sulphur,  graphite,  talc,  magnesite,  fluorite,  and  apatite. 

Halite,  or  rock  salt  is  mined  from  strata  deep  below 
the  surface  in  New  York,  Michigan,  Ohio,  Kansas,  Louis- 
iana, and  elsewhere.  It  is  frequently  obtained  by  drilling 
wells  down  to  the  bed  of  salt,  running  in  water  which  dis- 
solves the  salt,  then  pumping  out  the  water  and  evaporat- 
ing it.  At  Syracuse,  N.  Y.,  the  salt  is  already  in  solution 
by  groundwater,  so  that  it  is  only  necessary  to  pump  out 
the  salt  water  and  evaporate  it.  In  places  in  Utah, 
Nevada,  and  southern  California,  salt  occurs  in  great 
abundance  on  the  surface,  ready  to  be  gathered  up  and 
utilized.  In  some  localities  it  is  mined  like  coal  from 
underground  workings.  In  some  places  it  is  obtained  by 
evaporating  the  sea  water.  It  is  distinguished  from  all 
other  minerals  by  its  taste.     (See  fig.  179.) 


252 


PHYSICAL  GEOGRAPHY 


Gypsum  is  the  sulphate  of  lime  combined  with  the 
water  of  crystallization.  H=2,  luster,  pearly  to  dull. 
Compare  with  calcite.  When  heated  enough  to  drive  off 
some  of  the  water  it  forms  the  plaster  of  Paris.     It   is 


Pig.  180.  Gypsum  quarry,  Lyndon,  N.  Y.  The  upper  15  feet  are  limestone. 
The  lower  part  (60  feet)  of  the  quarry  consists  of  gypsum.  It  is  quarried 
for  use  in  the  manufacture  of  Portland  cement,  wall  plaster,  and  land 
plaster. 


used  in  making  wall-plaster,  stucco  work,  as  a  fertilizer 
for  soil  and  in  the  manufacture  of  Portland  cement. 
Alabaster,  a  variety  of  gypsum,  is  used  for  ornamental 
purposes.  Gypsum  occurs  in  beds  separated  by  layers  of 
shale  and  associated  with  salt  beds  ii^  many  places.  It  is 
quarried  in  New  York,  Michigan,  Kansas,  Iowa,  and  many 
of  the  more  western  states.     (Fig.  180.) 

Sulphur  is  obtained  in  Utah,  Nevada,   California,  and  Louis- 
iana, but  much  of  that  used   in  the  United   States  is   imported 


THE  LAND 


253 


from  the  Island  of  Sicily.     It  is  used  for  making  matches,  gun- 
powder, and  sulphuric  acid  and  as  a  disinfectant. 

Graphite,  sometimes  called  black  lead,  is  a  soft,  black  mineral 
composed  of  nearly  pure  carbon.  It  occurs  in  the  Adirondack 
Mountains,  N.  Y.,  and  in  Pennsylvania,  and  in  several  of  the 
western  states,  but  the  best  quality  is  imported  from  the  island 
of  Ceylon.  It  is  used  in  the  manufacture  of  lead  pencils,  cruci- 
bles, paint,  stove  polish  and  as  a  lubricant.     (Fig.  181.) 


Fig.  181.      Interior  of  Dixon's  graphite  mine,  four  miles  west  of  Hague,  N.  Y. 
The   graphite   is   scattered   through   the   rock   which   is    quarried,    crushed, 
•     and  the  graphite   separated.      (U.    S.   Geol.   Survey.) 


Talc  is  mined  in  St.  Lawrence  county,  N.  Y.,  in  Virginia, 
Pennsylvania,  New  Hampshire,  and  Vermont.  It  is  composed 
of  the  hydrous  silicate  of  magnesia,  is  one  of  the  softest  (H=l) 
of  all  the  minerals,  and  has  a  characteristic  greasy  or  soapy  feel. 
It  is  used  as  a  filler  in  paper  manufacture;  the  soapstone  variety 
is  used  for  switch  boards  in  electrical  work,  table  tops  in  chem- 
ical laboratories,  for  household  purposes  it  is  used  for  sinks, 
laundry  tubs,  cake  griddles,  foot  warmers,  etc.     Compare  speci- 


254  PHYSICAL  GEOGRAPHY 

mens  of  soapstone  with  foliated  talc,  describing  the  differences. 

Magnesite,  the  carbonate  of  magnesia,  is  used  in  making  car- 
bonic acid  for  soda  fountains,  as  a  filler  for  paper,  and  for  lining 
furnaces.  It  is  quarried  to  some  extent  in  California  but  much 
of  that  used  in  the  United  States  is  imported. 

Phosphates.  The  phosphate  of  lime  used  so  extensively  as  a 
fertilizer  consists  of  the  mineral  apatite.  The  purer  mineral 
form  is  quarried  in  Canada  and  the  more  massive  rock  form  is 
quarried  in  Florida,  South  Carolina,  Tennessee,  Alabama  and 
elsewhere. 

Fluorite.  Fluorite  is  the  mineral  formed  by  the  chemical 
union  of  fluorine  and  calcium.  It  crystallizes  in  cubes  and 
octahedrons,  but  it  cleaves  more  commonly  into  octahedrons; 
the  color  is  generally  green  or  purple,  but  it  is  sometimes  color- 
less. It  is  used  as  a  furnace  flux  and  for  the  manufacture  of 
hydrofluoric  acid  which  etches  glass. 

Four  of  the  lime  minerals  are  but  one  degree  apart  in  the 
scale  of  hardness  and,  being  common  minerals,  are  usually 
selected  as  types  in  the  scale:  gypsum,  2;  calcite,  3;  fluorite,  4; 
apatite,  5. 

Besides  the  minerals  mentioned  above,  there  are  a  hundred 
or  more  common  ones  somewhat  widely  distributed,  a  number 
of  them  having  some  economic  importance.  There  are  also 
more  than  a  thousand  that  are  much  less  common  and  many  of 
them  exceedingly  rare. 

Make  a  list  of  the  minerals  you  have  studied  with  the  char- 
acteristic properties  of  each. 

ROCKS 

The  minerals,  either  singly  or  in  various  combinations, 
make  up  most  of  the  rocks  on  the  exterior  of  the  earth. 
Some  rocks,  as  limestone,  or  serpentine,  are  composed  of 
a  single  mineral,  while  others,  as  granite  or  diabase,  are 
composed  of  several  different  ones.  In  the  glassy  vol- 
canic rocks  there  are  no  separate  minerals,  although  they 
consist  probably  of  fused  minerals  which  in  the  glass  have 
lost  their  identity. 

Rocks  are  commonly  grouped  in  three  general  classes, 


THE  LAND 


255 


based  on  origin,  namely,  sedimentary,  igneous,  and  meta- 
morphic. 

209.  Sedimentary  rocks  are  formed  by  the  accumula- 
tion of  sediments  in  water,  and  are  therefore  stratified. 
Included  in  this  class  are  certain  wind-formed  deposits 
that  might  be  distinguished  as  eolian.  The  sedimentary 
rocks  are  divided  into  the  following  groups  based  on  the 
chemical  composition  of  the  rock  mass: 


Fig.  182.  Micro-photograph  of  brown  sandstone.  The  white  particles  are 
nearly  all  fragments  of  quartz.  The  black  part  is  iron  oxide,  which  gives 
the  red  or  brown  color  to  the  rock  and  acts  as  a  cement  to  bind  the  sand 
grains  together. 

1.  Siliceous.  Most  of  the  sand  and  gravel  deposits 
are  siliceous  and  largely,  sometimes  entirely,  composed  of 
ground-up   fragments  of  quartz.     Along  with  the  quartz 


256 


PHYSICAL  GEOGRAPHY 


grains  there  are  frequently  variable  quantities  of  frag- 
ments of  other  common  minerals.  In  sandstones  the 
grains   are    cemented    together   by   some    substance,    most 


Fig.  183.  Brecciated  liinef-tom-,  Ilighgato  Falls  Vt.  The  angular  fragments 
of  limestone  are  held  together  by  calcite.  A  rock  in  which  the  fragments 
are    rounded   is   called   conglomerate.      (U.    S.    Geol.    Survey.) 

commonly  clay,  iron  oxide,  calcite,  or  silica;  sometimes 
two  or  more  of  these  substances  may  act  as  cement  in  the 
same  rock.  In  the  process  of  weathering  of  sandstones, 
the  cementing  substance  is  the  first  to  give  way  and  when 
destroyed  the  sandstone  crumbles  to  sand,  from  which  it 
was  first  formed.     (See  fig.  182.) 

Pebbles  or  gravel  may  be  cemented  in  the  same  way 


THE  LAND 


257 


and  by  the  same  means  as  the  sand  and  form  conglomerate 
or  puddingstone.  If  instead  of  the  rounded  pebbles  of 
the  conglomerate,  the  fragments  are  angular,  like  broken 
rock,  and  cemented  together,  it  forms  a  breccia. 


Fig.  184.  Limestone  quarry  in  the  Allegaany  Mountains  at  Bellefonte,  Pa. 
The  limestone  layers  are  nearly  vertical  due  to  the  folding  of  the  strata 
in  the  uplift  of  the  mountains.  The  limestone  is  here  used  for  making 
quicklime  and  as  a  furnace  flux  in  smelting  iron  ores.  In  other  places 
it  is  quarried  for  building  stone.  One  of  the  best  limestones  for  build- 
ing purposes  is  quarried  extensively  in  Indiana. 


2.  Argillaceous.  The  argillaceous  or  clayey  rocks  in- 
clude the  beds  of  clay  or  mud  as  well  as  the  hardened 
forms  of  these  which  form  the  shale  beds.  Clay  may 
grade  imperceptibly  into  shale  and  this  in  turn  through 
shaly  sandstones  into  sandstones  or  through  calcareous 
shale    into    limestone    and    by    metamorphism    into    slate. 

17 


258 


PHYSICAL  GEOGRAPHY 


Some  of  the  varieties  of  clay  are  china  clay,  trick  clay, 
potters  clay,  and  fire  clay. 

3.  Calcareous.  The  calcareous  rocks  are  composed 
of  the  minerals  calcite  and  dolomite,  and  include  the  many 
varieties  of  limestone  and  marble.  Some  of  the  common 
varieties  of  limestone  are  shell  limestone;  coral  limestone; 


Fig.  185.  Exposure  of  a  bituminous  coal  bed  near  Columbus,  Nevada.  The 
man's  hand  is  on  the  top  of  the  coal  seam  at  the  contact  with  the  over- 
hanging shale.  Coal  is  a  sedimentary  rock,  and  the  most  important  one  of 
the  fuels.      See  also  Figs.  69   and  231.      (U.   S.   Geol.   Survey.) 

chalk;  travertine,  including  the  stalactites  and  the  stalag- 
mites of  the  caves,  and  the  tufa  deposits  about  springs; 
hydraulic  limestone  or  waterlime;  marl;  and  lithographic 
limestone.  Gypsum,  the  sulphate  of  lime,  might  also  be 
added  to  the  calcareous  group.    (See  figs.  184,  177  and  180.) 

4.  Carbonaceous.'  The  carbonaceous  rocks  include  those 
composed  of  carbon  and  the  hydrocarbon  compounds,  as  bitu- 
minous and  anthracite  coal,  lignite,  peat,  asphalt,  petroleum,  and 
natural  gas.     (Fig  185.) 


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259 


Ferruginous  rocks  include  the  great  beds  of  iron  ore. 

Saline  rocks  include  the  beds  of  rock  salt. 

Alkaline  rocks  include  the  borax  and  soda  deposits  occurring 
in  arid  districts. 

210.  Igneous  rocks  may  be  divided  into  the  crystal- 
line or  granitoid  division,  sometimes  called  plutonic;  and 


Fig.  186.  Granite  Quarry,  near  Barre,  Vt.  Compare  with  FiGS.  184  and  185. 
Granite  contains  joint  planes  but  it  is  not  stratified  like  limestone  and 
coal.      (C.    H.   Richardson.) 

the  volcanic  or  glassy  and  stony  division,  to  which  is  some- 
times added  a  third  or  intrusive,  sometimes  called  porphy- 
ritic,  class. 

The  granitoid  rocks  include  those  that  cool  slowly 
under  pressure,  hence  they  are  crystalline  and  occur  only 
in  large  masses  that  were  formed  deep  below  the  surface 
and  are  now  exposed  because  of  the  erosion  of  the  over- 


260  PHYSICAL  GEOGRAPHY 

lying  rock.     They  are  composed  of  masses  of  interlocking 
crystals  of  different  kinds.     The  granitoid  rocks  are : 

1.  Granite  which  consists  of  quartz,  orthoclase  feld- 
spar, and  one,  two,  or  all  three  of  the  minerals,  mica, 
hornblende,  and  augite.  The  micas  are  more  common  in 
granite  than  the  other  two.     (Fig.  186.) 

2.  Syenite,  which  consists  of  orthoclase  and  horn- 
blende, augite  or  mica.  It  differs  from  granite  in  the 
absence  of  quartz. 

3.  Diorite  is  composed  of  plagioclase  feldspar,  and 
hornblende,  and  thus  differs  from  syenite  in  the  presence 
of  plagioclase  in  place  of  orthoclase. 

4.  Gahhro  is  composed  of  plagioclase,  augite,  and 
commonly  magnetite  and  olivine.  It  differs  from  diorite 
in  having  augite  in  place  of  hornblende.  It  is  darker 
colored  than  diorite,  which  in  turn  is  generally  darker 
than  granite  and  syenite.  Diabase  is  more  finely  crystal- 
line than  gabbro.  Basalt  which  .is  still  finer  grained, 
belongs  to  the  volcanic  or  intrusive  class.  The  last  two 
form  most  of  the  trap  rock  which  is  used  so  extensively 
for  making  good  roads.  One  of  the  best  known  exposures 
is  in  the  Palisades  on  the  Hudson. 

The  principal  volcanic  rocks  are:  (1)  olsidian,  volcanic 
glass;  (2)  pumice,  rock  froth,  the  very  porous  material  from  the 
surface  of  a  volcanic  outflow;  (3)  amygdaloid,  the  vesicular  or 
coarsely  porous  form  with  the  vesicles,  (holes  formed  by  the 
escaping  gas),  filled  with  other  minerals;  (4)  trachyte  and  (5) 
andesite  are  the  two  principal  stony  varieties;  (6)  porphyry  con- 
sists of  a  fine  matrix  with  imbedded  crystals;  (7)  tufa  or  tuif  is 
composed  of  fragments  such  as  volcanic  ashes  or  cinders  par- 
tially cemented  or  hardened. 

Pumice  is  used  for  grinding  and  polishing,  numbers  4,  5,  6, 
and  7  are  used  for  building  stone,  and  some  varieties  of  porphyry 
form  a  valuable  ornamental  stone. 

211.    Metamorphic   rocks  are  formed  from  either  sedimentary 


THE  LAND 


261 


or  igneous  rocks  by  a  process  known  as  metamorphism,  the  chief 
agents  of  which  appear  to  be  water,  heat,  and  pressure.  Marble 
is  metamorphic  limestone  and  is  more  crystalline  and  generally 
harder  and  brighter  colored  than  the  original  limestone.  Slate 
is  metamorphic  clay  or  shale  rendered  much  harder,  stronger. 


Fig.  187.  Crumpled  gneiss,  a  highly  metamorphosed  rock.  The  metamor- 
phism is  caused  in  large  part  by  the  lateral  pressure  which  produced  the 
wrinkling  of  the  layers.  Metamorphic  rocks  commonly  occur  in  regions 
of  folded  rocks,  that  is,  in  mountains  or  the  eroded  remnants  of  moun- 
tains.    (U.    S.    Geol,    Survey.) 


and  finely  crystalline.  Anthracite  is  thought  to  be  a  metamor- 
phic form  of  bituminous  coal  produced  by  pressure  and  heat. 
Quartzite  is  a  metamorphic  sandstone  with  silicious  cement. 
Other  metamorphic  rocks  are  gneiss,  serpentine,  and  schists  of 
many  kinds.     (Fig.  187.) 

Selected  specimens  of  the  different  rocks  should  be  carefully 
studied  and  compared,  and  all  rocks  found  in  the  field  trips 
should  be  named  and  classified.     In  many  places  in  the  northern 


262  PHYSICAL  GEOGRAPHY 

United  States  specimens  of  nearly  all  the  rocks  described  above 
may  be  obtained  from  the  deposits  left  by  the  glacier. 

SOIL 

Soil  is  produced  from  the  different  rocks  by  a  variety 
of  processes  known  as  weathering.  The  principal  wea- 
thering agencies  are  heat  and  cold,  moisture,  vegetation, 
wind,  and  the  chemical  and  mechanical  effects  produced 
by  these. 

212.  Disintegration  of  Rocks.— The  heating  of  the 
rock  under  the  rays  of  the  sun  causes  it  to  expand,  often 
enough  to  produce  cracks  and  the  breaking  off  of  frag- 
ments. The  sudden  cooling  of  the  heated  rock  by  a  shower 
of  rain  may  produce  fracture  by  contraction.  Freezing 
of  water  held  in  pores  and  cavities  in  the  rock  breaks  and 
fractures  it  by  the  expansion  of  the  ice  crystals. 

The  chemical  action  of  the  rain  water  and  the  ground 
water  breaks  up  hard  minerals,  such  as  feldspar  and  mica, 
causing  them  to  crumble  into  clay-  and  sand.  Portions 
of  the  limestone  are  dissolved  leaving  other  portions  in  fine 
fragments  in  the  soil. 

The  roots  of  plants  penetrate  cavities,  cracks,  and* 
joints  in  the  rocks  and  expanding  in  growing,  act  like 
wedges  to  split  the  rock  asunder.  Both  the  living  and 
decaying  plants  furnish  organic  acids,  which,  like  other 
acids,  act  on  the  minerals  to  dissolve  portions  of  them  and 
cause  the  remainder  to  crumble  to  fragments. 

"Winds  carry  sand  and  dust  against  the  hard  rock, 
grinding  off  the  surface.  This  agency  is  most  conspicuous 
in  dry  seasons  and  in  dry  climates. 

Gravity  acts  as  a  disintegrating  agent  especially  on 
cliffs  and  steep  slopes  by  pulling  down  boulders  and  frag- 
ments, causing  a  further  breaking  and  crushing  in  the 
falling,  and  at  the  same  time  exposing  fresh  surfaces  to 


THE  LAND  263 

the  action  of  the  weather.  Gravity  carries  down  large 
quantities  of  loose  mantle  rock  in  the  landslide  or  some- 
times by  a  very  slow  process  known  as  creep.  (See  figs. 
188  and  189.) 

Streams  grind  away  the  solid  rock  along  their  courses 
and   distribute   the    finely   ground-up   material   over   the 


Fig.  188.  View  near  Columbia,  Pa.,  showing  creep  due  to  the  action  of 
gravity  on  weathered  slate.  In  the  upper  half  of  the  picture  the  vertical 
layers  are  broken  and  creeping  down  the  hill  to  the  right.  (U.  S.  Gteol. 
Survey. ) 

flood  plain,  and  delta,  forming  alluvium  or  alluvial  soil. 
The  fine  sediment  carried  by  the  streams  into  lakes  settles 
on  the  lake  bottom  and  after  the  disappearance  of  the  lake 
forms  lacustrine  soil. 

Glaciers  in  cold  climates  are  active  agents  in  breaking, 


264 


PHYSICAL  GEOGRAPHY 


grinding,  and  disintegrating  rock  material  and  transport- 
ing it  to  other  Jocalities. 

213.     Mantle   Rock.— All   of   the    loose  material,   fine 
and  coarse,  that  covers  the  solid  bed  rock  produced  by  any 


Fig.  x89.  Creep  in  glacial  clay  soil  due  to  gravity.  THe  ciay  when  saturated 
with  water  in  a  wet  season  is  liable  to  creep  down  the  hillside,  as  shown 
in  the  photo.  Sometimes  on  a  steep  slope  a  large  mass  breaks  loose  and 
descends  rapidly  as  a  landslide.      (E.  R.  Smith.) 


of  the  above  agencies,  is  known  as  mantle  rock,  which 
everywhere  rests  upon  the  solid  massive  rock  similar  to 
that  exposed  in  the  different  rock  quarries  and  on  the  face 
of  rock  cliffs. 

The  change  from  solid  rock  to  soil  is  frequently  a 
gradual  one  in  which  there  is  no  sharp  line  of  separation 
between  the  two.  In  fig.  190  the  surface  is  fine  soil  resting 
upon  the  subsoil  which  contains  partially  disintegrated  rock 


THE  LAND 


265 


fragments  increasing  in  number  and  size  down  to  the  solid 
rock  at  the  bottom  of  the  quarry. 

The  thickness  of  the  mantle  rock  varies  greatly  in  different 
places.  In  many  places  on  steep  hillsides  there  is  none;  in 
other  places  it  is  as  much  as  three  or  four  hundred  feet  deep. 


Fig.  190.  Peerless  slate  quarry,  Caml)ria,  Md.  Showing  the  change  from  soil 
at  the  surface  through  partially  disintegrated  rock  to  the  fresh  unweathered 
slate  at  bottom.      (Maryland   Geological  Survey.) 

In  the  city  of  Washington  the  granite  rock  is  disintegrated  to  a 
depth  of  more  than  80  feet.  In  places  in  Brazil,  South  America, 
the  mantle  rock  is  400  feet  deep.  Such  great  thicknesses  as 
that  are  not  known  in  cold  climates  except  where  the  material 
has  been  transported  as  along  stream  courses  in  filled  valleys, 
alluvial  fans,  talus  slopes,  jor  deposits  made  by  the  glacier. 

Mantle  rock  changed  to  soil.  The  surface  portion  of 
the  mantle  rock  which  has  been  subject  to  weathering 
agencies  longest,  is  more  minutely  divided  and  broken  up 


266 


PHYSICAL  GEOGRAPHY 


than  the  underlying  portions.  The  completely  decayed  sur- 
face portion  which  supports  plant  growth,  is  called  soil  and 
the  partially  disintegrated  underlying  material  is  subsoil. 


^*^j*= 


Fig.  191.  Weathered  surface  of  granite  in  Oklahoma.  The  mantle  rock  is 
carried  away  by  wind  and  rain  as  rapidly  as  disintegration  takes  place. 
(U.   S.  Geol.    Survey.)      Compare  with  Figs.   192   and  193. 

The  most  productive  soils  contain  more  or  less  decaying 
vegetable  matter  or  humus.  Probably  an  important  con- 
stituent of  productive  soils  is  the  bacteria  or  microscopic 
organisms  which  hasten  the  decay  of  the  dead  plant  and 
the  growth  of  the  living  one. 

Soils  are  not  very  productive  until  they  contain  some  humus 
or  organic  material.  It  may  be  questioned  whether  the  mantle 
rock  is  truly  soil  until  it  receives  this  admixture  of  organic  mat- 
ter. This  mixing  of  the  organic  with  the  inorganic  is  accom- 
plished in  several  ways.  The  roots  of  many  plants  extend  to 
considerable  depths  and  leave  their  substance  to  decay  on  the 
death  of  the  plant.  Ants,  earthworms,  and  many  other  burrow- 
ing animals  carry  vegetable  material  down  and  fresh  rock  mater- 
ial to  the  surface,  thus  mixing  and  fertilizing  the  soil.  The 
farmer  assists  in  this  process  by  the  use  of  the  plow  and  the 
cultivator  with  which  he  mixes  the  organic  with  the  inorganic. 


THE  LAND 


267 


214.  Varieties  of  Soil.— The  body  of  nearly  all  the 
soils  is  made  up  of  clay  and  sand.  If  the  sand  is  absent 
it  is  clay  soil,  liable  to  be  cold  and  wet.  The  bogs  are  al- 
ways on  clay  soils.  If  the  clay  is  entirely  absent  the  land 
will  be  sandy,  dry,  and  crops  are  liable  to  be  burnt  out  by 
the  sun. 


Fig.  192.  Hunter  ore  bank,  Center  Co.,  Pa.  Residual  soil  on  limestone. 
Part  of  the  soil  has  been  removed  to  obtain  the  limonite  iron  ore  which 
is  mixed  through  it.  Remnants  of  the  limestone  show  in  the  view  project- 
ing through  the  mantle  rock. 

There  are  many  grades  of  soil  between  pure  clay  at  one  ex- 
treme and  pure  sand  on  the  other.  Probably  the  best  soil  is 
produced  by  a  somewhat  equal  mixture  of  the  two  called  loam  or 
loamy  soil.  The  soil  in  which  the  sand  prevails  is  best  adapted 
to  root  crops  like  potatoes  and  carrots.  The  clayey  soils  are 
better  suited  to  grass  and  similar  plants,  and  are  generally 
improved  by  either  surface  or  tile-draining. 

Along  with  the  clay  and  sand  is  a  small  percentage  of  the 
elements  that  give  fertility  to  the  soil.  The  plant  derives  the 
greater  part  of  its  food  from  the  atmosphere,  but  some  of  it 
comes  from  the  soil.  The  nitrates,  phosphates,  lime,  potash, 
and  other  alkalies,  along  with  organic  matter,  give  fertility  to 


268 


PHYSICAL  GEOGRAPHY 


the  soil  by  their  presence  or  render  it  barren  by  their  absence. 
The  decaying  vegetation  and  the  work  of  earth  worms  and  other 
animals  are  important  if  not  essential  elements  of  fertility. 

In  many  places  the  soil  becomes  poor  and  ceases  to  produce 
good  crops  because  of  the  loss  of  one  or  more  of  the  above  con- 
stituents. The  fertility  may  be  renewed  by  the  addition  of 
manure  or  some  of  the  commercial  fertilizers. 


Fig.  193.  Residual  soil  resting  on  marble,  Chester,  Co.,  Pa.  Part  of  the  soil 
has  been  removed  in  order  to  quarry  the  marble.  The  surface  of  the  soil 
is  nearly  a  level  plain,  but  the  rock  surface  underneath  is  quite  irregular. 
The  rock  surface  underneath  the  soil  in  Fig.   192  is  still  more  irregular. 

The  black  lands  in  many  places  are  composed  of  a  soil  that 
is  nearly  all  vegetable  material  with  sometimes  a  commingling 
of  animal  remains.  This  is  the  mucic  which  is  partially  decayed 
vegetation  and  which  forms  a  soil  very  rich  in  the  elements  of 
fertility,  but  one  lacking  in  body. 

Residual  soil  occurs  in  the  place  occupied  by  the  orig- 
inal rock  from  which  it  was  formed.     (Fio^s  192  and  193.) 


THE  LAND  269 

Transported  soils  have  been  carried  from  the  place  of 
disintegration  by  some  agency  as  water,  wind,  or  ice.  The 
principal  kinds  of  transported  soil  are : 

1.  Alluvial  soils  which  are  among  the  most  produc- 
tive of  all  soils.  There  is  the  happy  commingling  of  sand 
and  clay  base  with  the  elements  of  fertility,  all  of  which 
are  renewed  in  the  periodic  overflow  of  the  stream. 

2.  Lacustrine  soils  which  are  formed  on  former  lake 
beds  and  are  of  three  kinds:  (a)  The  black  muck  such  as 
the  black  lands  of  New  York,  formed  by  vegetable  accumu- 
lations generally  in  small  lakes;  (b)  the  fine  muds  washed 
from  the  lands  and  filling  in  the  lake  basin.  The  great 
wheat  lands  of  the  northwest  belong  to  the  second  class, 
(c)  Marls  formed  largely  of  remains  of  small  shell  animals. 

3.  Glacial  soil.  Most  of  the  soil  of  the  northern 
United  States  is  glacial  soil  and  it  varies  greatly  in  qual- 
ity. The  average  productiveness  of  the  whole  area  has 
been  greatly  increased  by  the  glacial  action,  which  mixes 
the  soil  from  different  localities.  In  some  places,  the  soil 
is  very  poor  because  of  too  much  clay,  too  much  sand,  or 
too  many  boulders,  but  the  average  is  much  better  than  in 
regions  of  similar  rocks  where  the  soil  has  not  been  mixed 
by  the  glacier. 

215.  Life  History  of  Sedimentary  Rocks.— The  dffferent 
changes  which  rocks  undergo  have  already  been  mentioned.  It 
remains  now  simply  to  connect  these  stages  in  the  history  of 
the  rocks  to  see  that  the  rock  material  is  moving  in  cycles  on 
the  earth  much  as  the  Water  is  doing. 

As  soon  as  the  rocks  are  exposed  on  the  surface,  the  weather- 
ing agencies  begin  the  work  of  disintegration.  Some  of  the 
materials  go  into  solution  and  are  quickly  carried  back  to  the 
sea.  Other  portions  are  broken  up  into  fragments  large  and 
small,  which  are  washed  by  the  rains  into  streams  and  carried 
into  the  sea  or  lake,  where  they  form  beds  of  gravel,  sand,  and 
clay.      In  time  these  are  changed  to  beds  of  conglomerate,  sand- 


270  PHYSICAL  GEOGRAPHY 

stone,  and  shale  or  slate,  and  elevated  above  the  water  to  be 
again  attacked  by  the  weathering  agencies  and  go  once  more 
through  a  similar  circuit. 

Similarly  the  limestone  beds  on  the  continents  are  taken  into 
solution  by  the  groundwater  and  through  the  springs  poured 
into  the  streams  and  thence  carried  to  the  sea  where  they  are 
taken  up  by  the  corals,  crinoids,  shell  fish,  etc.,  whose  accumu- 
lated remains  again  form  great  bedded  deposits  of  limestone 
which  are  lifted  above  the  sea  level  to  begin  a  similar  cycle  over 
again. 

The  sand  grains  and  the  mud  particles  in  a  recently-formed 
rock  may  have  formed  part  of  an  indefinite  number  of  similar 
beds  of  rock  in  the  past.  When  a  bed  of  rock  is  weathered,  dis- 
integrated, and  carried  away,  the  material  is  only  changed  in 
positron  but  not  destroyed.  The  first  rock  may  disappear  as  a 
rock  but  the  material  reappears  elsewhere,  forming  parts  of 
other  rocks. 

Hardening  of  the  Rocks.  The  induration  or  harden- 
ing of  the  material  into  solid  rock,  as  the  sand  to  sand- 
stone, lime-mud  to  limestone,  mud-beds  to  shale,  is  caused 
partly  by  the  pressure  of  overlying  materials,  partly  by 
horizontal  pressure,  and  largely  by  cementing  materials 
deposited  between  the  grains  by  the  circulating  ground- 
water. The  induration  may  take  place  before,  during,  or 
after  the  elevation,  and  in  some  instances  never  takes 
place  at  all,  as  in  the  case  of  soft  clays  that  occur  in  the 
midst  of  the  hard  rocks  of  ancient  geological  periods. 

216.  The  Geographic  Cycle.— The  life  history  of  a 
land  area— the  geographic  cyisle— cycle  of  erosion— or  to- 
pographical cycle,  refers  to  the  successive  changes  in  the 
surface  features  of  an  area  from  the  time  it  is  elevated 
above  sea  level  until  it  is  worn  down  to  sea  level  again, 
preparatory  to  beginning  a  new  cycle. 

Since  the  development  of  a  river  system  is  at  the  ex- 
pense of  the  land  area  which  it  drains,  the  stages  of  the 


THE  LAND  271 

land-area's  history  are  similar  to  those  of  the  river  history 
previously  described. 

Youth.  Many  land  areas,  when  first  elevated,  have  a 
fairly  level  surface,  which  is  soon  made  irregular  by  the 
development  and  trenching  of  numerous  valleys  over  the 
area.  This  incision  or  deep  trenching  of  the  streams  dis- 
tinguishes the  very  early  youthful  stage  of  the  cycle  from 
the  final  old  age  stage  of  the  preceding  cycle  before  the 
uplift.  The  youthful  stage  is  further  characterized  by 
broad  stretches  of  undrained  areas  between  the  streams, 
frequently   containing  many   lakes   and   swamps. 

Maturity.  As  the  cycle  advances  from  youth  towards 
maturity,  tributaries  to  the  stream  develop  in  great  num- 
bers until  all  the  inter-stream  areas  are  drained.  In  the 
mature  stage  (study  the  Charleston,  W.  V.,  sheet)  the 
valleys  are  numerous  and  deep.  Consequently  the  hill- 
sides are  steep  and  likely  to  have  many  bold,  rocky  cliffs 
and  steep  talus  slopes.  The  divides  consist  of  narrow 
ridges  devoid  of  lakes  and  swamps. 

Old  Age.  The  area  passes  from  maturity  into  old  age 
and  again  approaches  a  plain  in  regularity,  but  a  lowland 
plain  of  old  age  instead  of  the  upland  plain  of  youth. 
The  tops  of  the  hills  are  lowered  by  erosion,  the  talus 
slopes  extend  to  the  tops  of  the  hills  and  spread  out 
farther  at  the  base.  As  the  tops  of  the  hills  are  worn 
down,  the  level  of  the  valley  plain  rises,  the  two  approach- 
ing a  common  level.  Finally  the  harder  and  more  resistant 
rocks  which  do  not  erode  so  rapidly,  stand  up  as  knobs 
or  prominences,  called  monadnocks,  on  the  nearly  level 
plain-area  called  a  peneplain  (pene,  almost).   (See  fig.  194.) 

The  erosion  continues  on  the  peneplain  until  the  entire 
area  is  brought  to  base  level,  and,  in  fact,  does  not  wholly  cease 
until  it  is  brought  to  sea  level;  but  the  erosion  during  the  pene- 
plain stage   proceeds   with   such   comparative    slowness,   that   in 


272  PHYSICAL  GEOGRAPHY 

most  cases  the  topography  of  the  peneplain  is  overtaken  by  one 
of  the  movements,  either  of  depression,  which  causes  drowning 
by  carrying  it  below  the  sea,  or  of  elevation,  which  causes  re- 
juvenation or  renewal  of  youth. 


Fig.  194.  Mt.  Pony,  south  of  Culpepper,  Va.  A  monadnock  on  a  peneplain. 
The  surrounding  rocks  have  been  worn  down  to  a  lower  level  than  the 
harder,  more  resistant  rocks  which  form  the  monadnock  mountain.  (U. 
S.   Geol.    Survey.) 

The  length  of  the  geographic  cycle  is  determined  by  a  num- 
ber of  more  or  less  complex  factors,  such  as: 

(1)  The  initial  elevation,  whether  great  or  small,  whether  it 
is  accompanied  by  metamorphic  change  or  not.  Thus  an  ele- 
vation of  metamorphic  rocks  to  great  altitudes  like  the  Rocky 
and  Sierra  Nevada  Mountains,  will  take  a  much  longer  period 
of  time  to  be  reduced  to  base  level  than  a  low  elevation  of  soft 
material,  like  the  coastal  plain  of  New  Jersey  or  Maryland. 

(2)  The  vigor  of  the  eroding  agents  which  is  determined 
largely  by  climatic  conditions,  such  as  amount  arid  distribution 
of  rainfall  and  changes  of  temperature.  An  area  in  which  the 
rainfall  is  concentrated  in  heavy  showers  will  be  eroded  much 
more  rapidly  than  one  in  which  the  rain  is  more  evenly  dis- 
tributed throughout  the  year. 


THE  LAND  273 

The  annual  rain  fall  in  the  Bad  Lands  of  Western  Nebraska 
is  much  less  than  in  the  fertile  districts  of  Eastern  Nebraska. 
Yet  the  erosion  is  much  greater  because  the  rain  is  concentrated 
in  a  few  heavy  showers  separated  by  long  intervals  of  drouth. 

(3)  The  resistance  offered  by  the  rocks — for  example,  beds 
of  unconsolidated  sand  and  clay  may  pass  through  the  youthful 
and  mature  stages  to  old  age  in  a  small  fraction  of  the  time  re- 
quired by  the  hard  crystalline  rocks;  in  fact,  the  cycle  may  be 
completed  in  the  first  case  before  it  is  scarcely  begun  in  the 
second. 

The  length  and  stage  of  the  cycle  are  clearly  not  a  question 
of  years  at  all.  A  pile  of  soft  earth  in  the  laboratory  under  a 
vigorous  shower  may  be  made  to  pass  through  all  the  stages  of 
an  erosion  cycle  in  a  few  hours,  while  an  area  of  hard  rocks 
might  take  many  hundreds  of  thousands  of  years. 


BEFEBENCES 

Dana,  Manual  of  Mineralogy  and  Lithology,  Wiley  &  Sons. 

Kemp,  A  Study  of  the  Rocks,  Scientific  Publishing  Co.,  New 
York. 

Crosby,  Common  Minerals  and  Rocks. 

Howell,   Washington    School    Collection   of  Rocks   and   Min- 
erals. 
Soils: 

Shaler,  Origin  and  Nature  of  Soils,  12th  Annual  Report 
U.  S.  Geological  Survey,  Part  1.  Also  the  last  chap- 
ter in  "Aspects  of  the  Earth"  by  the  same  author. 

Hilgard,  Soils  and  Their  Properties,   Macmillan  Co. 

Hilgard,  Relations  of  Soil  to  Climate,  Bull.  No.  3,  Weather 
Bureau,  U.  S.  Department  of  Agriculture. 

Whitney,  Some  Physical  Properties  of  Soils,  Bull.  No.  4, 
Weather  Bureau,  U.  S.   Department  of  Agriculture. 

Whitney,  Soil  Fertility,  U.  S.  Department  of  Agriculture, 
Farmer's  Bulletin  No.  257. 

Many  of  the  other  publications  of  the  Department  of  Agri- 
culture contain  valuable  information  on  the  subjects  of  soils, 
and  would  be  a  great  aid  to  the  teacher  in  this  subject. 


18 


CHAPTER  VIII 

PHYSIOGRAPHIC  AGENCIES 

Diastrophic  Movements,  Volcanoes,  Earthquakes 

Among  the  most  active  and  important  physiographic 
agencies  are  those  of  rainfall,  weathering,  and  the  work 
of  streams  and  waves  described  in  previous  chapters. 
However,  one  can  readily  see  that  the  continued  action  of 
these  eroding  agencies  on  the  upland  areas  would  in  time 
carry  all  the  mountains  and  plateaus  to  sea  level  unles-i 
some  new  force  or  forces  should  work  in  opposition  to  ele- 
vate new  land  areas  or  re-elevate  old  ones  from  time  to 
time.  Such  a  force  exists  in  the  heated  interior  of  the 
earth  and  is  manifested  in  volcanoes  and  in  the  elevation 
and  depression  of  plains,  plateaus,  and  mountains.  The 
force  is  shown  in  several  ways:  there  is  (1)  Diastrophism, 
a  very  slow  movement  affecting  large  areas;  (2)  Vulcan- 
ism,  a  rapid  outpour  of  material  from  the  interior  of  the 
earth  to  the  surface  through  volcanoes;  (3)  Seismic  move- 
ments, the  uplift  and  depression  of  large  areas  by  the 
force  which  produces  earthquakes. 

These  three  phenomena  are  possibly  more  or  less  re- 
lated to  each  other,  yet  each  may  act  independently. 

217.  Diastrophism  '(literally,  a  twisting  or  warping) 
is  the  term  used  to  designate  the  movements  of  large  por- 
tions of  the  earth's  crust  and  includes  both  the  movements 
of  elevation  and  depression.  An  upward  movement  of 
any  portion  of  the  earth 's  surface  is  probably  accompanied 
by  a  considerable  depression  elsewhere;  that  is,  the  sur- 
face is  warped  or  twisted  by  the  action  of  the  internal 

274 


PHYSIOGRAPHIC    AGENCIES  275 

forces.  Diastrophism,  includes  both  epeirogenic  and 
orogenic  uplifts.  An  uplift  of  a  plateau  in  which  there  is 
little  or  no  disturbance  of  the  strata  is  called  an  epeiro- 
genic movement  in  contradistinction  to  an  orogenic  move- 
ment in  which  the  strata  are  folded  and  wrinkled.  The 
epeirogenic  movement  produces  plains  and  plateaus  in 
which  the  strata  are  horizontal  or  but  slightly  inclined. 
The  orogenic  movement  produces  mountain  ranges  in 
which  the  strata  are  folded,  wrinkled  and  frequently- 
broken. 

Evidence  of  elevation  and  depression.  That  many  of  the  plains, 
plateaus,  and  mountains  have  been  elevated  to  their  present 
position  by  some  dynamic  force  and  have  not  always  been  at 
this  level  is  proven  by  the  great  numbers  of  fossil-animal  and 
plant  remains  of  organisms  that  live  only  in  the  sea,  showing 
that  these  rocks  were  formed  on  the  sea  bottom  and  then  raised 
to  their  present  height.  In  many  places  in  the  rocks  there  are 
fossil  ripple  marks  that  were  made  in  the  shallow  water  of  the 
sea. 

On  the  coasfal  plain  at  Pozzuoli  near  Naples,  the  Romans 
erected  a  temple  to  Jupiter  Serapis.  Later  the  land  was  de- 
pressed by  diastrophism  until  the  building  was  nearly  sub- 
merged in  the  Mediterranean  Sea.  This  plain  was  afterwards 
elevated  above  sea  level,  and  both  the  elevation  and  depression 
were  so  gradual  as  not  to  overthrow  the  temple,  three  columns 
of  which  are  still  standing.  They  show  the  effect  of  their 
former  submergence  in  the  sea  by  the  borings  of  the  Lithodomi, 
a  species  of  rock-boring  shells  that  live  in  the  Mediterranean 
Sea.  Here  is  a  positive  historic  example  of  a  depression  of  a 
coastal  plain  25  feet  or  more  and  re-elevation  of  the  same,  all  in 
a  period  of  about  2,000  years.  Many  other  examples,  historic 
and  geologic,  might  be  cited  of  both  elevation  and  depression. 
Can  the  student  mention  any  from  his  own  observation  or 
reading?     (See  fig.  195.) 

218.  Cause  of  Crustal  Movements.— The  cause  of 
the  diastrophism  is  thought  to  be  the  shrinkage  of  the  in- 
terior of  J:he  earth  due  to  the  loss  of  heat.     Except  from 


276 


PHYSICAL  GEOGRAPHY 


changes  due  to  the  seasons,  the  surface  rocks  have  a  nearly 
uniform  temperature,  but  the  interior  of  the  earth  which 
has  a  much  higher  temperature  is  thought  to  be  cooling, 


Fig.  195.  Remains  of  a  Roman  temple  near  Pozzuoli,  Italy.  Since  the  temple 
was  erected  about  2000  years  ago,  the  area  has  been  below  sea  level  and 
the  three  marble  columns  on  the  right  were  perforated  by  rock-boring 
molluscs.  A  few  feet  at  the  base  of  the  columns  were  buried  in  the  mud 
and  hence  not  perforated. 

and  as  it  cools  it  grows  smaller.  The  outer  crust,  which 
is  not  losing  heat  and,  consequently,  not  shrinking,  must 
settle  down  on  the  decreasing  interior  portion  which  causes 
depressions  and  elevations  over  the  surface. 

Another  cause  probably  acting  conjointly  with  the  pre- 
ceding, is  the  extrusion  or  transfer  of  solid,  liquid  and 
gaseous  material  and  of  heat  from  the  interior  of  the  earth 
to  the  surface  through  fissures  and  volcanoes. 

219.    Results  of  the  Crustal  Movement.— The    results 


PHYSIOGRAPHIC    AGENCIES  277 

of  the  crustal  warpings  are  the  depressions  of  great  seg- 
ments of  the  crust,  forming  the  ocean  basins,  and  the  ele- 
vation of  other  portions,  forming  the  great  continental 
land  masses  or  portions  of  the  same,  the  crumpling  and 
elevation  of  mountain  chains,  the  elevation  and  depres- 
sion of  plains  and  plateaus. 

The  elevations  may  be  apparent,  not  real.  That  is, 
if  the  ocean  bottom  should  sink  deeper  toward  the  center 
of  the  earth  than  the  continental  masses,  the  effect  would 
be  the  same  as  if  the  land  masses  were  elevated.  There 
are  some  very  perplexing  problems  connected  with  the 
origin  of  the  continents  and  ocean  basins. 

220.  Isostacy.— The  principle  of  isostacy  assumes  that  the 
surface  portions  of  the  earth  are  in  temporary  equilibrium  due 
to  equality  of  gravitative  pressure.  That  is,  the  continents, 
plateaus,  and  mountains,  stand  above  sea  level  because  they  are 
lighter  than  rocks  beneath  the  ocean  bed  which  are  heavier  and 
hence  depressed.  A  disturbance  of  this  equilibrium  by  mov- 
ing a  large  quantity  of  material  from  one  portion  of  the  surface 
to  another  causes  corresponding  movements  of  elevation  and 
depression.  The  erosion  of  the  rocks  from  the  continent  and 
the  deposition  of  the  material  in  the  margin  of  the  sea  causes 
a  rising  of  the  continent  because  of  the  removal  of  the  land, 
and  a  sinking  of  the  marginal  sea  bottom  because  of  the  addi- 
tional load.  The  movement  continues  until  isostatic  equilibrium 
is  again  established  or   some  other  force  or  agency   intervenes. 

The  great  movements  which  approximately  fixed  the  position 
of  the  ocean  basins  and  the  continental  masses,  probably  took 
place  very  early  in  geological  history.  But  since  that  time, 
changes  of  level  less  extensive  have  been  going  on  from  time 
to  time  which  tend  to  modify  the  outline  and  surface  features 
of  the  land  areas. 

221.  Changes  in  the  Shore  Line.— The  ocean  basins 
are  now  overflowing,  and  the  overflow  laps  up  over  the 
border  of  the  continents  on  the  continental  shelf.  The 
shore  line,  or  the  meeting  of  the  land  and  sea,  may  be  ex- 


278 


PHYSICAL  GEOGRAPHY 


pected  to  shift  from  time  to  time  owing  to  several  differ- 
ent causes : 

(1)  Primarily  the  warping  or  diastrophic  movements 
described  above. 


Lfff^fS^Wkl^J^^J^mm.         I^KT, 

. 

i'lG.  196.  General  view  of  Mt.  Vesuvius  from  across  the  Bay  of  Naples.  City 
of  Naples  at  base  of  the  mountain.  Eruption  of  1872.  Note  the  great 
volumes  of  steam  from  the  center  of  the  mountain  and  from  the  streams 
of  lava  on  the  sides.  Torrents  of  rain  descend  from  the  condensing 
vapors. 

y2)  The  cutting  away  of  the  land  by  the  waves  on  the 
shores  and  the  consequent  advance  of  the  sea  on  the  land. 
(Chapter  VL) 

(3)  Filling-in  of  the  sea  bottom  by  the  material  car- 
ried from  the  land  by  the  rivers. 

(4)  The  infilling  from  the  materials  thrown  out  by 
volcanoes  in  the  sea  and  from  the  accumulated  organic 
deposits. 


PHYSIOGRAPHIC    AGENCIES 


279 


VOLCANOES 

Volcanic  eruptions  are  among  the  most  vivid  and  im- 
pressive phenomena  of  nature.  The  effect  produced  by 
them  on  the  surface  features  of  the  earth  are  much  less 
than  those  produced  by  the  erosive  agencies,  yet  because 
of  the  greater  intensity  of  the  volcanic  forces  manifest 
for  a  short  period  of  time,  they  make  a  much  stronger  im- 
pression on  the  mind  of  the  observer. 


Fig.  197.  Near  the  summit  of  Mt.  Vesuvius  previous  to  the  great  eruption 
of  1906,  showing  the  surface  of  a  lava  flow  in  foreground,  the  great 
ash  cone  composed  of  fragments  of  lava  thrown  out  of  the  crater,  and  a 
small  cinder  cone  formed  on  the  surface  of  a  lava  stream  on  the  left.  This 
part  of  the  mountain  was  destroyed  in  the  recent  eruption. 

222.  Mt.  Vesuvius.— Mt.  Vesuvius,  one  of  the  best 
known  of  all  active  volcanoes,  was  considered  to  be  extinct 
at  the  beginning  of  the  Christian  era.  In  the  year  79 
A.  D.,  there  was  a  violent  eruption  which  threw  out  vast 


280  PHYSICAL  GEOGRAPHY 

quantities  of  fine  fragments  and  water  vapor  which  con- 
densed as  rain  and  fell  in  torrents.  The  city  of  Pompeii 
was  deeply  buried  under  the  ashes  and  dust  of  the  erup- 
tion from  which  it  is  now  being  excavated  by  the  Italian 
government.  Herculaneum  was  buried  at  the  same  time 
by  the  ashes  and  torrents  of  mud  formed  by  the  heavy 
rainfall  with  the  ashes.     (See  figs.  196,  197  and  198.) 


Fig.  198.  Crater  of  Mt.  Vesuvius,  a  few  years  previous  to  the  great  eruption 
of  1906.  Part  of  a  second  crater  is  visible  encircling  the  inner  one. 
There  was  a  part  of  a  third,  not  shown  in  the  picture. 

Following  the  great  eruption  of  '79,  Mt.  Vesuvius  remained 
quiet  for  many  years,  the  next  outburst  occurring  in  203,  the 
next  in  472,  again  in  512,  993,  1036,  1049,  1138,  and  1139,  after 
which  it  remained  quiet  for  nearly  500  years.  But  during  this 
period  of  quiescence  volcanic  activity  was  manifest  in  the  smaller 
volcanoes  in  the  vicinity. 

In  1631  Vesuvius  again  became  violently  active,  no  less  than 


PHYSIOGRAPHIC    AGENCIES  281 

seven  streams  of  lava  flowing  out  from  the  crater,  partially 
destroying  the  villages  of  Ressina,  Portici,  and  Torre  del  Greece. 
In  1737  there  was  a  stream  of  lava  from  the  mountain  estimated 
to  contain  300,000,000  cubic  feet. 

Probably  one  of  the  most  violent  eruptions  of  the  mountain 
since  1737  was  the  recent  one  in  April,  1906,  when  the  vast  quan- 
tity of  ashes  thrown  out  destroyed  the  village  of  Ottajano,  while 
great  stre.ams  of  lava  extended  into  and  partially  destroyed  the 
village  of  Boscotrecasse.  Many  lives  were  lost  and  a  great  deal 
of  property  destroyed. 

Besides  the  many  violent  eruptions  of  Mt.  Vesuvius  since 
'79  and  in  the  preceding  ages,  there  has  been  a  great  deal  of 
volcanic  activity  in  the  region  surrounding  the  mountain,  some 
of  which  is  recent  and  some  in  times  prehistoric.  Monte  Nuovo, 
a  symmetrical  volcanic  cone,  was  built  upon  the  coastal  plain 
in  three  days'  time  in  September,  1538.  The  Solfatara  near  by 
has  been  emitting  sulphur  and  arsenic  fumes  and  carbon  dioxide 
for  centuries. 

On  the  island  of  Ischia,  there  are  12  volcanic  cones,  one  of 
which  is  now  utilized  as  a  harbor  for  small  vessels.  The  city 
of  Sorrento  on  the  other  side  of  the  Bay  of  Naples  is  built  on 
part  of  a  volcanic  crater. 

223.  Mt.  Pelee,  Martinique.— There  are  numerous 
volcanic  mountains  on  several  of  the  West  Indies.  In 
fact,  many  of  these  islands  are  composed  of  volcanic  rocks, 
but  previous  to  1902  the  volcanoes  were  thought  to  be  ex- 
tinct. The  last  eruptions  had  been  in  1718  and  1812  and 
had  been  forgotten.  In  1851  there  were  earthquakes  on 
Martinique  and  some  fine  ashes  were  thrown  out  of  Mt. 
Pelee. 

On  April  25,  1902,  a  great  cloud  of  smoke  poured  out 
of  Mt.  Pelee  and  for  several  days  there  were  rumbling 
noises  accompanied  by  steam  and  clouds  of  dust.  On  May 
3,  there  was  an  eruption  which  destroyed  a  sugar  factory 
at  the  base  of  the  mountain  and  killed  a  number  of  peo- 
ple. At  7:50  A.  M.,  May  8,  occurred  the  eruption  that 
proved  the   most    destructive   to   human    life    of   any    in 


282 


PHYSICAL  GEOGRAPHY 


America.  The  city  of  St.  Pierre  with  its  population  of 
30,000,  17  ships  in  the  bay,  and  all  the  country  places  be- 
tween the  E-oxelane  and  the  Riviere  Blanche  were  des- 


Fia.  199.  yiew  of  Mt.  Pelee,  Martinique,  some  months  after  the  great  erup- 
showing  the  spine  from  a  distance.  Notice  the  absence  of  vegetation  in  the 
vicinity  of  the  mountain.      (E.  O.  Hovey,  Am.  Mus.  Nat.  Hist.) 

troyed  almost  instantaneously  by  an  explosion  which  shot 
a  great  volume  of  hot  gas  and  dust  from  the  top  of  the 
mountain  over  the  doomed  city  and  harbor.  (See  figs. 
199  to  202.) 

On  July  9th,  there  was  another  eruption,  thought  to 
be  similar  to  the  one  that  destroyed  the  city  of  St.  Pierre. 
The  following  graphic  account  is  given  by  an  eye  witness* 
of  the  second  eruption: 

"As  the  darkness  deepened,  a  dull  red  reflection  was  Seen 
in  the  trade  wind  cloud  which  covered  the  mountain  summit. 
This   became   brighter   and  brighter,   and   soon   we   saw  red-hot 

*Tempest  Anderson  in   Smithsonian  Report,   1902,  p.   328. 


OF  / 

PHYSIOGRAPHIC    AGENCIES 


283 


stones  projected  from  the  crater  bowling  down  the  mountain 
slopes,  and  giving  off  glowing  sparks.  Suddenly  the  whole 
cloud  was  brightly  illuminated  and  the  sailors  cried,  'The  moun- 
tain bursts ! '  In  an  incredibly  short  space  of  time  a  red-hot 
avalanche  swept  down  to  the  sea.  We  could  not  see  the  sum- 
mit, owing  to  the  intervening  veil  of  cloud,  but  the  fissure  and 


Pig.  200.  St.  Pierre  and  Mt.  Pelee  after  the  eruption,  June,  1902.  View- 
looking  north.  Note  the  great  number  of  north- south  walls,  where  the 
east- west  ones  have  been  destroyed  by  the  blast  from  the  volcano.  (Am. 
Mus.  Nat.  Hist.) 

the  lower  parts  of  the  mountain  were  clear,  and  the  glowing 
cataract  poured  over  them  right  down  to  the  shores  of  the  bay. 
It  was  dull  red,  with  a  billowy  surface,  reminding  one  of  a 
snow  avalanche.  In  it  there  were  larger  stones  which  stood 
out  as  streaks  of  bright  red,  tumbling  down  and  emitting 
showers   of   sparks.    In   a   few   minutes    it    was   over.    A   loud. 


284 


PHYSICAL  GEOGRAPHY 


angry  growl  had  burst  from  the  mountain  when  this  avalanche 
was  launched  from  the  crater.  It  is  difficult  to  say  how  long 
an  interval  elapsed  between  the  time  when  the  great  glare 
shown  on  the  summit  and  the  incandescent  avalanche  reached 


Jb^ 

'^■Ml 

^^^■^ 

-4> 

Fig.  201.  Ejected  block  of  lava  thrown  out  of  Mt.  Pelee  Aug.  30,  1902. 
Photographed  March,  1903.  This  block  is  on  the  plateau  about  one  mile 
from  the  crater.      (Am.  Mus.  Nat.  Hist.) 


the  sea.  Possibly  it  occupied  a  couple  of  minutes;  it  could  not 
have  been  much  more.  Undoubtedly  the  velocity  was  terrific. 
Had  any  buildings  stood  in  its  path  they  would  have  been  utterly 
wiped  out,  and  no  living  creature  could  have  survived  that  blast.'* 
"The  most  peculiar  feature  of  these  eruptions  is  the  ava- 
lanche of  incandescent  sand  and  the  great  black  cloud  which 
accompanies  it.  The  preliminary  stages  of  the  eruption,  which 
may  occupy  a  few  days  or  only  a  few  hours,  consist  of  outbursts 
of  steam,  fine  dust,  and  stones,  and  the  discharge  of  the  crater 
lakes  as  torrents  of  water  or  as  mud.  In  them  there  is  noth- 
ing unusual,  but  as  soon  as   the  throat  of  the  crater  is  thor- 


PHYSIOGRAPHIC    AGENCIES 


285 


oughly  cleared  and  the  climax  of  the  eruption  is  reached,  a  mass 
of  incandescent  lava  rises  and  wells  over  the  lip  of  the  crater 


Fig.  202.  Near  view  of  the  hijge  spine  protruding  from  the  crater  of  Mt. 
Pelee.  Height  about  1200  feet  above  the  rim  of  the  crater.  View  on 
March  25,   1903.      (See  Fig.  199)    (Am.  Mus.  Nat.  Hist.) 


286  PHYSICAL  GEOGRAPHY 

in  the  form  of  an  avalanche  of  red-hot  dust.  It  is  a  lava  blown 
to  pieces  by  the  expansion  of  the  gases  it  contains.  It  rushes 
down  the  slopes  of  the  hill,  carrying  with  it  a  terrific  blast 
which  mows  down  everything  in  its  path.  The  mixture  of  dust 
and  gas  behaves  in  many  ways  like  a  fluid.  The  exact  chemical 
composition  of  these  gases  remains  unsettled.  They  apparently 
consist  principally  of  steam  and  sulphurous  acid.  There  are 
many  reasons  which  make  it  unlikely  that  they  contain  much 
oxygen,  and  they  do  not  support  respiration." 

A  unique  feature  of  the  activity  of  Mt.  Pelee  was  the 
growth  in  the  crater  of  a  monstrous  spine  or  monolith  of 
volcanic  rock  which  extended  about  1200  feet  above  the 
rim  of  the  crater  (see  fig.  202)  at  its  maximum.  This  great 
spine  was  an  object  of  absorbing  interest  and  considerable 
speculation  on  the  part  of  the  observers. 

224.  Soufriere.— Mt.  Soufriere  on  the  island  of  St.  Vincent 
became  violently  active  about  the  same  time  as  Mt.  Pelee  and 
on  May  7,  1902,  a  great  eruption  of  this  mountain  destroyed  a 
large   amount   of   property   and  many   lives. 

225.  Krakatoa.— In  1883,  on  the  island  of  Krakatoa, 
in  the  East  Indies,  occurred  one  of  the  most  violent  vol- 
canic eruptions  known  to  mankind.  For  many  weeks 
previous  there  had  been  great  disturbance  by  earthquakes 
and  eruptions  of  vapor  and  dust.  In  such  quantities  was 
this  dust  thrown  into  the  air  that  for  100  miles  around 
the  island  the  darkness  of  midnight  prevailed  at  midday. 

At  10  o'clock  Monday  morning,  Aug.  27,  came  the  cul- 
mination of  this  disturbance  in  what  was  probably  the 
loudest  noise  that  has  ever  been  heard  on  this  earth,  a 
noise  recognized  almost  3,000  miles  away.  Two-thirds  of 
the  island  was  blown  into  the  air,  some  of  it  to  a  height  of 
17  miles,  and  some  of  it  pulverized  so  finely  that  it  was 
three  years  or  more  before  it  all  settled  out  of  the  atmos- 
phere. Where  part  of  the  island  stood  before  the  explo- 
sion, the  sea  was  1000  feet  deep  afterwards.     The  enorm- 


PHYSIOGRAPHIC    AGENCIES  287 

ous  force  exerted  in  such  an  eruption  is  almost  beyond 
human  comprehension. 

226.  Definition. — A  volcano  proper  is  a  pipe-like  or 
chimney-like  opening  in  the  earth's  crust  through  which 
molten  rock,  rock  fragments,  vapor,  or  gases  escape  from 
the  interior  to  the  surface.  Much  of  the  solid  material 
so  ejected  is  commonly  deposited  around  the  opening,  thus 
building  up  a  cone-shaped  mountain.  The  volcanic  cone 
has  a  basin  or  funnel-shaped  depression,  the  crater,  at  the 
top  which  leads  into  the  pipe  or  neck  of  the  volcano, 
through  which  the  materials  are  ejected.  The  cone  is  not 
an  essential  part  of  a  volcano,  but  is  generally  a  product 
of  one.     (Fig.  203.) 

V 


Fig.   203.      Ideal   section   of  volcanic    cone,      a,    Crystalline    rocks,      b,    c,    Sedi- 
mentary rocks.     V,  Crater  of  volcano,     s,  Remnant  of  a  former  crater. 

227.  Volcanic  Cones. — The  cones  are  of  three  types:  (1) 
They  may  be  composed  of  volcanic  ashes,  cinders,  or  lapilli,  ma- 
terials which  have  been  blown  out  in  a  fragmental  condition 
and  much  of  which  falls  around  the  mouth  of  the  opening, 
building  up  an  ash  cone  as  steep  as  the  fragmental  material  will 
lie.  Mt.  Nuovo  near  Naples  is  an  example.  (2)  The  material 
may  come  out  through  the  opening  quietly  and  flow  away  in 
streams  or  sheets,  sometimes  a  great  many  miles.  The  shape 
of  the  cone  in  this  case  depends  on  the  amount,  and  the  tem- 
perature of  the  material  ejected,  but  generally  it  has  a  very 
gentle  slope  on  the  exterior,  almost  flat  compared  with  the  ash 
cone.  This  may  be  called  the  lava  cone  of  which  the  Hawaiian 
volcanoes,  Kilauea  and  Mauna  Loa,  are  type  examples.  (3)  The 
eruptions  may  vary  in  the  same  volcano  at  different  times;  at 
one  time  lava  may  flow  out  in  streams,  and  again  be  blown  out 
in  fragments;  the  cone  will  be  built  up  in  part  of  one  and  in 
part  of  the  other,  forming  a  mixed  lava  and  ash  cone,  Vesuvius 
and  Etna  are  examples. 


288  PHYSICAL  GEOGKAPHY 

228.  Phenomena  of  a  Volcanic  Eruption.— The  dif- 
ferent kinds  of  eruptions  might  be  grouped  into  two  classes, 
the  explosive  and  the  non-explosive  or  quiet,  with  numer- 
ous modifications  of  each.  Both  classes  are  frequently 
but  not  always  preceded  by  rumblings  and  earthquakes, 
which  continue  sometimes  for  several  months  before   a 


Fig.  204»  Driblet  cones  built  of  scoria  and  volcanic  bombs  on  the  lava  field 
at  Cinder  Butte,  Idaho.  Tumuli  or  small  volcanic  cones.  (See  also  Fia. 
197.)      (U.  S.  Geol.  Survey.) 

great  eruption.  In  the  explosive  type  these  disturbances 
are  apt  to  increase  in  intensity  up  to  the  time  of  explosion. 
The  eruption  is  frequently  preceded  by  the  escape  of  puffs 
of  steam  and  other  gases,  all  terminating  finally  in  a  tre- 
mendous explosion  which  generally  destroys  the  top  of 
the  volcanic  cone. 

In  the  non-explosive  eruption  there  is  a  gradual  swell- 
ing up  and  rising  of  the  lava  in  the  crater  until  it  over- 
flows the  rim  and  descends  the  cone  in  one  or  more  great 


PHYSIOGRAPHIC    AGENCIES  289 

streams.  In  the  great  eruption  in  Iceland  in  1783,  two  of 
these  streams  flowed  down  the  valley  forty-live  and  fifty 
miles  respectively  from  the  source.  In  the  Hawaiian  vol- 
canoes, the  streams  sometimes  flow  to  the  edge  of  the  island 
and  pour  the  molten  lava  into  the  sea. 

In  the  very  high  volcanic  mountains  the  pressure  of  the 
great  vertical  column  of  lava  in  the  crater  is  sufficient  at  times 
to  burst  the  cone  and  form  one  or  more  openings  in  the  side, 
through  which  the  lava  pours  out,  building  up  new  cones. 
Sometimes  great  cracks  or  fissures  form  in  the  side  of  the  cone 
through  which  the  lava  flows,  and  after  hardening,  forms  dikes, 
cutting  the  sides  of  the  cone.  Sometimes  on  the  surface  of  the 
lava  streams  or  floods  there  are  small  cones  or  craterlets  built 
up,  through  which  gas  and  sometimes  lava  poured  out.  These 
are  driblet  cones.     (Fig.  204.) 

229.  Materials  Ejected.— The  materials  from  a  vol- 
cano consist  of:  (1)  gases  and  vapors;  (2)  solid  materials; 
(3)  molten  lava. 

The  gases  consist  of  water  vapor,  chlorine,  sulphur, 
carbon  dioxide,  carbon  monoxide,  arsenic  and  mercury, 
frequently  carrying  great  clouds  of  fine  dust  which  ap- 
pears like  smoke. 

The  solid  materials  consist  of  fine  fragments  called  dust 
or  ashes ;  small  pieces,  lapilli ;  large,  irregular  masses  torn 
from  the  neck  of  the  opening;  and  large,  rounded  masses 
somewhat  elongated  and  pointed,  called  volcanic  bombs. 
The  latter  are  formed  by  small  masses  of  lava  that  are 
thrown  out  while  molten,  cooling  in  the  passage  through 
the  air.     (Fig.  205.) 

Forms  of  lava.  The  hardened  lava,  free  from  gas  pores,  is 
obsidian,  which  is  generally  a  colored  glass  resembling  cinder 
or  slag  from  an  iron  furnace.  If  the  lava  contains  water  vapor 
and  other  gases  escaping  as  it  cools,  it  forms  vesicular  lava, 
named  from  the  vesicles  or  bubbles;  or  if  there  Is  an  excess  of 

19 


290 


PHYSICAL  GEOGRAPHY 


the  gas,  so  that  the  product  is  very  porous  and  light,  it  forms 
pumice.     Dark,  heavy  lava  forms   basalt  on  cooling. 

Tufa  and  dust.  The  fine  fragmental  material  is  sometimes 
cemented  by  the  percolating  waters,  forming  volcanic  tufa,  a 
soft,  porous  rock  that  is  used  extensively  for  building  stone  in 
central  and  southern  Italy,  and  to  some  extent  in  California. 
The  volcanic  dust  is  sometimes  carried  long  distances.  Large 
deposits  of  it  on  the  plains  of  western  Nebraska  are  supposed 


Fig.  205.  Elongated  volcanic  bomb,  13  feet  long,  on  the  lava  plains  at  Cinder 
Buttte,  Idaho.  The  bombs  are  generally  shorter  than  this  one.  (U.  S. 
Geol.  Survey.) 


to  have  blown  from  the  Rocky  Mountain  area  or  beyond.  Con- 
siderable falls  of  volcanic  dust  which  have  come  from  some 
distant,  but  perhaps  submarine  explosion,  are  sometimes  met 
by  vessels  at  sea.  A  volcanic  eruption  is  sometimes  accom- 
panied by  a  heavy  downpour  of  rain  caused  by  the  condensation 
of  the  ejected  vapors  which  falling  with  the  dust  and  ashes, 
forms  great  streams  of  volcanic  mud,  to  be  changed  to  tufa  as 


PHYSIOGRAPHIC    AGENCIES 


291 


the  mud  dries.  It  was  the  downpour  of  mud  that  overwhelmed 
Herculaneum,  while  Pompeii  was  buried  under  the  volcanic 
dust. 

230.     Commercial  Products  of  Vulcanism.— Some  of 

the  material  from  volcanoes  has  economic  importance :   ( 1 ) 


Fig,    206.      Volcanic   butte,    San   Lnis    Obispo,    Calif.      Remnant    of    a   volcanic 
mountain.     The  porphyritic  lava  is  quarried  for  building  stone. 


extensive  sulphur  deposits  occur  in  Sicily,  Italy,  and  Ice- 
land; (2)  the  pumice  is  used  for  grinding  and  polishing 
material;  (3)  lava  and  tufa  are  used  for  building  stone; 
(4)  the  fine  volcanic  ash  on  Mt.  Vesuvius  and  vicinity 
furnishes  excellent  soil  which  supports  many  flourishing 
vineyards;  (5)  lava  is  used  for  road-metal  in  some  places 
and  sometimes  as  ballast  for  railways;  (6)  Pozzuolana,  a 
fine  volcanic  ash,  is  used  in  making  cement  in  Italy;  val- 
uable ores  are  sometimes  found   in  volcanic   rocks.     The 


292 


PHYSICAL  GEOGRAPHY 


Fig.  207.  Muir's  Butte,  Cal.  A  volcanic  cone  on  which  eroding  agencies  have 
done  little  work.  Note  the  symmetry  of  the  cone.  (Detroit  Publishing 
Co.) 


PHYSIOGRAPHIC    AGENCIES  293 

rich  gold  miines  at  Cripple  Creek  and  in  the  San  Juan 
Mountains  are  in  volcanic  rocks. 

231.  Active  and  Extinct  Volcanoes.— Some  volcanoes 
are  always  active,  some  quiet  for  years,  and  some  for  cen- 
turies. The  first  are  called  active,  the  last  are  commonly 
classed  as  extinct,  and  those  in  the  second  class  are  doj-- 


Fia.  208.     Mt.    Shasta,    California.      A  volcanic   mountain  more  deeply   eroded 
than  the  preceding.      (Detroit  Publishing  Co.) 

mant.  It  is  not  always  possible  to  tell  when  a  volcano 
passes  from  the  dormant  to  the  extinct  class.  In  fact,  some 
that  were  thought  to  be  extinct  have  become  active,  Mt. 
Vesuvius  was  supposed  to  be  extinct  previous  to  the  great 
eruption  in  the  year  79  which  destroyed  Herculaneum  and 
Pompeii.  Mt.  Pelee  had  not  been  active  since  1851  until 
the  great  eruption  in  1902. 

There  are  300  active  volcanoes,  that  is,  300  have  been 
active  during  recent  years.  How  many  of  the  so-called 
extinct  and  dormant  ones  may  become  active  at  any  time 
is  not  known. 


294 


PHYSICAL  GEOGRAPHY 


232.  Effect  of  Erosion  on  Volcanoes.— The  eroding  agencies 
are  always  at  work  on  the  volcanic  cones,  even  while  they  are 
erupting,  but  each  eruption  generally  obscures  the  effects  of 
previous  erosion.  After  activity  ceases  the  effects  of  erosion 
are  soon  manifest.  The  cone  is  dissected  by  streams  and  car- 
ried away  through  radiating  valleys  much  like  any  other  moun- 


FiG.  209.  Devil's  Tower,  Wyoming.  Remnant  of  an  eroded  volcanic  moun- 
tain. By  some  this  is  thought  to  be  the  remnant  of  a  laccolite  (Fig.  211)  ; 
others  consider  it  the  throat  of  a  volcano  from  which  the  surrounding  ash 
cone  has  been  eroded.      (U.  S.  Geol.  Survey.) 


tain  peak.  (Mt.  Shasta  should  be  studied  as  a  type  of  volcanic 
cone  in  youthful  stage  of  erosion.  See  Folio  2,  Top.  Atlas,  U.  S. 
Geol.  Surv.,  study  figs.  207,  208,  and  209.) 

Sometimes   the  cone   is   composed  largely  of  ashes  and   the 
throat-opening  is   filled  with   rock  cooled  from  a   molten   state. 


PHYSIOGRAPHIC    AGENCIES  295 

In  such  a  case  the  softer  exterior  will  be  eroded  rapidly,  and 
leave  the  harder,  more  resistant  rocks  of  the  neck  or  plug  stand- 
ing as  a  prominent  elevation.     (See  fig.  209.) 

233.  Calderas. —  Sometimes  the  bottom  of  the  crater 
subsides  as  activity  ceases,  leaving  a  large  and  often  deep 
depression,  called  a  caldera,  which  when  filled  or  partly 
filled  with  water  forms  a  crater  lake.  Crater  Lake  near 
Mt.  Hood  in  Oregon  is  an  example.  (Study  Crater  Lake 
topographic  sheet  and  explanation  in  Folio  2  of  Top 
Atlas.)      (See  figs.  75  and  76.) 

Part  of  the  calderas,  at  least,  are  thought  by  some  to 
be  formed  by  the  bottom,  and  part  of  the  rim  being  blown 
away  by  a  violent  explosion  instead  of  by  subsiding. 
Sometimes  an  eruption  terminates  the  existence  of  a  crater 
lake,  as  was  the  case  on  Mt.  Pelee  where  the  small  lake 
that  was  there  previous  to  the  eruption  was  entirely 
destroj^ed. 

234.  Distribution  of  Volcanoes.— The  greater  num- 
ber of  active  volcanoes  are  located  around  the  border  of 
the  Pacific  Ocean.  There  is  a  chain  of  them  extending 
from  Cape  Horn  at  the  extremity  of  South  America  along 
the  western  border  of  both  Americas,  across  the  Aleutian 
Islands  to  Asia,  and  down  the  Asiatic  coast.  There  are 
several  groups  in  the  Pacific  Ocean,  a  number  in  the  At- 
lantic Ocean,  some  on  Iceland  and  the  West  Indies,  and 
another  great  group  in  the  Mediterranean  Sea  around  the 
south  end  of  Italy. 

There  is  good  reason  for  thinking  that  there  are  many  vol- 
canic peaks  scattered  over  the  sea  bottom.  The  Hawaiian  Is- 
lands and  many  other  islands  are  but  the  tops  of  volcanic  moun- 
tains, built  up  on  the  sea  bottom,  while  no  doubt  there  are  many 
others  whose  tops  are  below  the  surface  of  the  sea. 

In  1867,  among  the  Tonga  Islands  in  the  Pacific,  a  shoal 
was  discovered  surrounded  by  water  6,000  feet  deep.  Ten  years 
later  steam  was   observed  rising  from  this    shoal  and   in  eight 


296  PHYSICAL  GEOGRAPHY 

years  more  there  was  an  island  of  volcanic  ashes  two  miles 
long  and  200  feet  high.  Unless  there  is  further  activity  the 
island  will  soon  be  cut  away  by  the  waves  and  again  form  a 
shoal. 

During  the  time  of,  or  immediately  following  the  San  Fran- 
cisco earthquake,  a  new  volcanic  island  appeared  in  the  Bogoslof 
group  among  the  Aleutian  Islands. 

235.  Life  History  of  a  Volcano.- The  life  history  of 
a  volcano  begins  with  some  changes  deep  below  the  surface 
and  beyond  our  observation.  Although  the  outbreak  is 
frequently  preceded  by  earthquakes,  the  first  visible  evi- 
dence is  a  crack  or  opening  extending  downward  indefinitely 
through  the  solid  rocks.  Through  this  opening  are  ejected 
gases,  molten  lava,  and  heated  rocks,  part  of  which  accu- 
mulate around  the  top  of  the  opening,  building  up  in  time 
a  cone-shaped  mountain.  This  period  of  up-building  may 
be  called  the  period  of  youth  and  growth.  The  volcano  in 
its  maturity  is  a  lofty  mountain  peak  which  finally  ceases 
to  erupt  and  becomes  extinct.  The  eroding  agencies  which 
have  been  overshadowed  in  this  upbuilding  process  now 
show  the  effect  of  their  activity  in  wearing  away  the  top 
and  sides  of  the  mountain.  The  first  step  is  to  carry  away 
the  soft  fragmental,  cinder  portion  of  the  cone,  leaving  the 
harder  central  core  or  neck  which  is  finally  worn  down  to, 
or  below,  the  level  of  the  original  area  on  which  it  started. 

236.  Fissure  Eruptions.— Sometimes  eruptions  take  place 
through  elongated  openings,  or  fissures,  instead  of  through 
chimney-like  rpenings  or  craters,  and  are  then  called  fissure 
eruptions.  They  do  not  build  craters,  but  spread  out  over  the 
adjoining  region  in  great  sheets  or  floods,  sometimes  many 
hundred  feet  thick.  Such  are  the  great  lava  fields  over  parts 
of  Washington,  Oregon,  Idaho  and  California,  covering  an  area 
of  200,000  square  miles,  thousands  of  feet  in  thickness. 

Ther  is  no  sharp  separation  between  eruptions  through 
craters  and  through  fissures.  In  fact,  small  fissure  eruptions 
occur  frequently  on  volcanic  cones  where  the  pressure  from  the 
interior  frequently  forms  cracks  or  fissures  in  the  side  or  base 


PHYSIOGRAPHIC    AGENCIES  297 

of  the  cone,  through  which  the  lava  may  flow  to  the  surface  in- 
stead of  overflowing  the  rim  of  the  crater.  The  hardening  of 
the  material  in  the  fissure  forms  a  dik?. 

Dikes  of  igneous  rocks  may  form  likewise  in  places  remote 
from  any  volcanic  cone,  such  as  those  in  the  city  of  Syracuse, 


Fig.  210,  Devil's  Slide,  Colorado.  A  dike  of  igneous  rock.  The 
central  portion  disintegrated  more  rapidly,  causing  the  de- 
pression. The  outer  portions  of  the  dike  are  more  durable 
causing  them  to  stand  up  as  walls  above  the  surface. 

N.  Y.,  and  in  the  vicinity  of  Little  Falls,  Ithaca,  and  elsewhere 
in  New  York  State,  in  many  places  along  the  Appalachian 
region,  also  around  Lake  Superior  and  in  many  other  localities. 
The  Palisades  on  the  Hudson  are  composed  of  igneous  rocks 
that  came  up  in  a  molten  state  through  great  fissures  or  were 
forced  out  between  layers  of  other  rocks.     (Fig.  210.) 

237.  Laccolites.— Sometimes  the  molten  material  rising 
through  fissures  does  not  reach  the  surface  but  pushes  up  the 
overlying  rock  and  spreads  out  between  the  layers  in  a  mushroom- 


298 


PHYSICAL  GEOGRAPHY 


shaped  mass  called  a  laccolite.  (See  fig.  211.)  It  is  called  a 
sheet  or  sill  if  it  spreads  out  between  the  strata  in  flat  sheets 
without  arching  the  overlying  rock. 

238.     Causes  of  Vulcanism.— There  are  some  features 
connected  with  volcanic  eruptions  that  are  not  well  under- 


FiG.  211.  Laccolites  in  Henry  Mountains,  Utah.  Upper  figure  shows  section 
through  laccolite  as  first  formed.  Lower  figure  shows  by  dotted  lines  part 
removed  by  erosion.      (After  Gilbert). 

stood.  The  source  of  the  heat  that  melts  the  rock  is  not 
known  certainly;  it  may  in  part  be  produced  by  pressure 
and  gravity,  in  part  by  chemical  action,  in  part  by  residual 
heat  of  the  earth.  The  force  that  lifts  the  huge  column  of 
molten  rock  10,000  feet  or  more  above  the  level  of  the  sea 
may  be  due  to  expansion  of  the  rocks  by  heat,  and  espe- 
cially the  expansion  of  the  gases  and  vapors  included  in 


PHYSIOGRAPHIC    AGENCIES  299 

them,  aided  by  the  gravitative  downward  pressure  of  sur- 
rounding heavier  material.  The  cause  of  violent  explo- 
sions that  blow  out  such  vast  quantities  of  fragmental 
material  is  probably  the  expansion  of  the  gases,  especially 
water  vapor. 

EARTHQUAKES 

Earthquakes  are  important  geographic  factors  in  their 
effect  on  topography,  on  life,  and  in  their  relation  to  vol- 
canoes. There  have  been  at  least  three  destructive  earth- 
quakes in  the  United  States  during  the  past  century  be- 
sides hundreds  of  minor  ones,  of  which  there  is  little  or  no 
record. 

239.  Mississippi  Valley  Earthquake,  1811.— The  first  of  the 
great  earthquakes  in  this  country,  of  which  there  is  any  written 
account,  began  in  the  Mississippi  Valley  between  St.  Louis  and 
Memphis  on  December  16,  1811,  and  continued  at  intervals  for 
several  months.  It  began  at  two  o'clock  in  the  morning,  when 
the  people  awoke  to  find  chimneys  falling,  furniture  thrown 
about  and  the  earth  rocking  and  trembling. 

"At  7  o'clock  a  rumbling  like  distant  thunder  was  heard  and 
in  an  in  *ant  the  earth  was  convulsed  so  that  no  one  could 
stand.  Looking  at  the  ground  the  terrified  people  saw  it  rise 
and  fall,  as  earth  waves  like  those  upon  the  sea  rushed  past, 
waving  the  trees  until  the  branches  interlocked,  and  causin,g 
yawning  cracks  to  open.  Giant  forest  trees  were  split  for  40 
feet  up,  half  standing  on  one  side  of  the  fissure,  the  remainder 
on  the  other.  Some  of  the  earthquake  rents  were  of  great 
size,  having  widths  of  30  feet  or  more,  while  some  are  reported 
as  many  as  five  miles  in  length.  Others  were  circular  in  form. 
Into  some  of  the  cracks  rushed  the  waters  from  swamps  and 
bayous,  while  elsewhere  small  streams  or  even  rivers  left  their 
old  beds  and  made  new  channels  through  the  cracks. 

"In  some  places  there  was  a  blowing  out  of  the  earth,  bring- 
ing up  coal,  wood,  sand,  etc.,  trees  being  blown  up,  cracked  and 
split,  and  falling  by  thousands  at  a  time. 

"Many  of  these  great  fissures  are  still  open  at  the  surface, 
and  steep  banks   formed  by  landslips   are  still  visible.     Several 


300 


PHYSICAL  GEOGRAPHY 


lakes  were  formed  on  river  bottoms.  Reelfoot  Lake,  in  western 
Tennessee,  formed  at  this  time,  is  five  miles  wide,  twenty-five 
miles  long  and  twenty-five  feet  deep. 


Fig.  212.     EartlKiuake  fissure   8  to    10   feet  deep  on  the  hill  east  of  Reelfoot 
Lake,  Teun.,  formed  m   1811.      (M.  L.  Fuller.) 


"During  the  three  months  following  December  16,  there 
were  recorded  in  the  Mississippi  Valley  1,874  earthquake 
shocks,  of  which  eight  were  violent,  ten  severe,  and  thirty-five 
alarming."*     (See  figs.  212  and  77.) 

240.  Charleston  Earthquake.- On  August  31,  1886, 
the  city  of  Charleston,  S.  C.,  and  the  surrounding  region 
were  severely  shaken  by  an  earthquake,  the  effects  of  which 
were  felt  as  far  north  as  New  England  and  as  far  west  as 
Minnesota.  Nearly  every  building  in  the  city  of  Charles- 
ton was  injured  to  some  extent,  many  of  them  severely  in- 

*M.  L.  Fuller,  Pop.   Sci.  Mon.,   July,   1906, 


PHYSIOGRAPHIC    AGENCIES 


301 


(50SEI5MAL3   OF  THE 

Ca\RLl  bTO\  £.\R1HQUAKE 

ROSSI   FORCLSCAL£ 
ScWkoTMUM 


Fig.  213.  Map  showing  area  of  disturbance  by  the  Charleston  Earthquake, 
1886.  The  concentric  lines  are  the  Isoseismals  which  converge  around  the 
centre,  near  Charleston.  The  intensity  was  greatest  near  Charleston,  and 
least  in  Vermont  and  Minnesota,  beyond  which  it  was  not  perceived. 
(U.  S.  Geol.  Survey.) 


302 


PHYSICAL  GEOGRAPHY 


jured  and  some  totally  demolished.  There  was  consider- 
able loss  of  property  and  life  in  the  area  surrounding 
Charleston.  Fissures  were  opened  in  the  earth,  springs 
ceased  flowing  in  some  places,  and  broke  out  in  other  places. 


Fig.  214.  San  Francisco  earthquake,  April,  1906.  Earthquake  fracture  near 
Olema,  California.  The  part  on  the  left  of  the  bank  has  been  thrust  away 
from  the  observer.  The  fence  was  built  after  the  earthquake.  (J.  F.  New- 
som.) 

railway  tracks  were  twisted  and  distorted  and  in  one  in- 
stance a  locomotive  was  thrown  from  the  track. 

241.  San  Francisco  Earthquake.— Every  year  there 
are  many  earthquakes  in  California,  but  most  of  them  are 
not  perceptible  to  the  senses  and  rarely  are  any  of  them 
very  destructive.  In  1872  an  earthquake  destroyed  the 
village  of  Lone  Pine  and  formed  a  great  fissure  through 
the  Sierra  Mountains  for  200  miles. 


PHYSIOGRAPHIC    AGENCIES 


303 


The  earthquake  which  destroyed  more  lives  and  prop- 
erty in  California  than  any  other  was  the  one  that  took 
place  in  the  early  morning  of  April  18,  1906.  There  were 
a  number  of  minor  shocks  at  intervals  for  several  days, 
but  nearly  all  the  damage  was  done  in  65  seconds  time. 


Tig.  215.  San  Francisco  earthquake.  On  the  railway  between  Los  Gatos  and 
Santa  Cruz.  The  rails  were  stretched  so  much  that  a  piece  had  to  be  cut 
out  before  the  road  could  be  straightened.      (J.  C.  Branner.) 


A  large  part  of  the  city  of  San  Francisco  was  destroyed 
by  the  earthquake  and  the  fires  which  were  caused  by  it. 
Great  damage  was  done  at  Santa  Rosa,  San  Jose,  Stanford 
University,  and  many  other  towns  in  the  area  of  destruc- 
tion. A  great  many  buildings  were  thrown  down  or  other- 
wise wrecked.  Nearly  all  chimneys  were  shaken  down. 
Water  pipes,  sewers  and  bridges  were  rent  apart.  Trees 
were  uprooted  in  large  numbers,  some  were  snapped  off, 
leaving  the   stumps   standing.     Fissures   were   opened   in 


304 


PHYSICAL  GEOGRAPHY 


the  earth  and  closed  again ;  in  one  place  it  was  reported 
that  a  cow  was  engulfed.  Line  fences  were  moved.  Roads 
and  railways  were  twisted  and  shifted.  In  one  place  the 
steel  rails  were  stretched  so  that  it  was  necessary  to  cut  out 
a  piece  several  inches  long  before  they  could  be  replaced. 
(See  fig.  215.) 


Fia.  216.  San  Francisco  earthquake,  A  vertical  drop  of  seven  feet  east  of 
Watsonville,  California.  The  foreground  was  on  a  level  with  the  orchard 
before  the  earthquake.  Note  the  craterlets  in  the  foreground  near  the 
break.      (J.  C.  Branner.) 

The  earthquake  was  caused  by  a  great  fissure  or  crack 
extending  375  miles  from  Point  Arena  on  the  north  to 
Mount  Pinos  on  the  south,  along  which  the  earth  was  frac- 
tured and  shifted  horizontally  a  distance  varying  in  dif- 
ferent places  from  six  to  twenty  feet.  The  area  of  destruc- 
tion following  this  line  extended  over  400  miles  long  and 
fifty  miles  wide,  but  the  shock  was  felt  as  far  north  as 


PHYSIOGRAPHIC    AGENCIES 


305 


Coos  Bay,  Oregon,  and  eastward  into  Nevada.  The  passage 
of  the  earthquake  was  recorded  by  delicate  instruments  as 
far  away  as  Sitka,  Alaska ;  Washington,  D.  C. ;  Tokio, 
Japan,  and  Potsdam,  Germany.  It  probably  passed  around 
the  globe     ^(Figs.  214  to  217.) 


Fiu.  217.  San  Francisco  earthquake.  View  on  a  road  across  alluvial  lands 
near  Salinas,  California.  The  stair-step  appearance  is  due  to  the  breaking 
and  sinking  down  along  the  fractures.  This  is  not  on  the  main  fracture  or 
fault  plane  which  produced  the  earthquake.      (J.  C.  Branner.) 

This  earthquake  was  probably  no  more  violent  than  the 
one  mentioned  in  1872  and  probably  others,  and  much  less 
violent  than  the  one  in  1811,  but  the  fact  that  it  occurred 
in  the  most  densely  populated  part  of  California  made  the 
destruction  of  human  life,  and  property  much  greater  than 
any  other  recorded  in  the  United  States. 

242.  Kingston  Earthquake.— There  have  been  a  great  many- 
destructive  earthquakes  in  Central  and   South  America  and  the 

20 


306  PHYSICAL  GEOGRAPHY 

West  Indies.  One  of  the  most  disastrous  in  recent  times  was 
that  which  destroyed  the  city  of  Kingston,  Jamaica.  The  first 
shock  came  at  3:30  P.  M.,  January  14,  1907,  and  was  followed  by 
fifteen  severe  shocks  during  the  following  week.  More  than  one 
thousand  persons  were  Rilled  and  nearly  every  building  in  the 
city  was  injured;  many  of  them  were  totally  destroyed. 

The  area  affected  by  the  Kingston  earthquake  was  much 
smaller  than  the  others  mentioned.  There  seems  to  have  been 
little  damage  done  outside  of  a  radius  of  ten  miles  from  the  city. 
The  old  city  of  Port  Royal,  just  across  the  bay,  was  severely  in- 
jured during  the  earthquake  in  1682,  when  part  of  the  city  sank 
beneath  the  ocean.  The  remnant  of  the  old  city  was  injured  in 
the  recent  quake. 

243.  Earthquake  of  Lisbon.— One  of  the  most  destructive 
earthquakes  recorded  in  human  history,  was  that  on  November 
1,  1755,  when  the  city  of  Lisbon  was  destroyed  and  over  60,000 
persons  perished  in  a  few  minutes.  It  began  with  noise  like 
heavy  thunder  which  was  immediately  followed  by  a  most  violent 
agitation  of  the  surface,  in  which  the  ground  rose  and  fell  like 
the  waves  of  the  sea;  the  neighboring  mountains  were  shaken 
like  reeds  and  rent  asunder  in  many  places.  Great  chasms 
opened  in  the  city  into  which  large  buildings  tumbled  and  dis- 
appeared from  view.  The  waters  of  the  ocean  retreated  at  first 
and  then  returned  in  a  great  sea  wave,  fifty  feet  high,  which 
rushed  over  the  doomed  city,  completing  the  ruin  caused  by  the 
shaking.  The  area  of  destruction  extended  as  far  away  as  the 
Alps  Mountains  and  across  into  northern  Africa  where  several 
villages  were  destroyed. 

Japan  is  now  the  foremost  nation  in  the  scientific  investiga- 
tion of  earthquake  phenomena.  It  was  aroused  to  action  by  the 
severe  earthquake  of  October,  1891,  in  which  7,000  people  were 
killed,  17,000  injured,  and  20,000  buildings  destroyed. 

244.  The  Cause  of  Earthquakes.— Anything  that  pro- 
duces a  violent  jar  or  concussion  in  the  earth  will  produce 
an  earthquake.  The  explosion  of  a  heavy  blast  in  a  mine 
causes  a  trembling  of  the  earth.  The  explosion  of  the 
chemical  works  near  San  Francisco  some  years  ago  was 
felt  forty  miles  away.  In  nature  the  following  agencies 
produce  earthquake  waves:    (1)  A  sudden  fracturing  of 


PHYSIOGRAPHIC    AGENCIES  307 

the  rocks  or  the  slipping  or  shifting  of  a  large  mass  of 
rock  along  a  fissure  or  fracture  in  the  earth's  crust  pro- 
ducing a  geological  fault;  (2)  the  explosion  of  a  large 
volume  of  gas  or  steam;  (3)  the  slumping  off  of  a  large 
mass  of  sediment  from  the  edge  of  the  continental  shelf 
into  the  deep  ocean  basin. 

245.  Distribution  of  Earthquakes.— Earthquakes  are 
common  in  volcanic  regions,  and  in  mountainous  countries, 
especially  in  young  and  growing  mountains.  They  are 
numerous  and  destructive  around  the  Pacific  Ocean,  fol- 
lowing the  volcanic  zone;  but  they  are  not  limifed  to  vol- 
canic regions  as  shown  by  the  fact  that  the  most  severe 
earthquake  recorded  in  the  United  States  took  place  in  the 
Mississippi  Valley  as  remote  from  mountains  and  volcanoes 
as  is  possible  in  this  country. 

Earthquakes  may  occur  anywhere  at  any  time,  but  they 
are  not  likely  to  be  either  severe  or  abundant  in  eastern 
or  central  United  States  in  comparison  with  the  Pacific 
border. 


BEFEBENGES 

Volcanoes : 

Russell,  Volcanoes  of  North  America,  Macmillan  &  Co.,  1897, 
$4. 

Hull,  Volcanoes  Past  and  Present,  Scribner's  Sons,  N.  Y., 
1892,  $1.50. 

Judd,  Volcanoes,  D.  Appleton  &  Co.,  N.  Y.,  1881,  $2. 

Bonney,  Volcanoes,  Putnam's  Sons,  N.  Y.,  1899,  $2. 

Geikie,  Ancient  Volcanoes  of  Great  Britain,  The  Macmillan 
Co.,  N.  Y.,  1897,   $11.25. 

Diller,  Mt.  Shasta,  National  Geographic  Monographs,  Amer- 
ican Book  Co.,  1895. 

Dana,  Characteristics  of  Volcanoes,  Dodd,  Mead  &  Co.,  N.  Y., 
1891,  $5. 

Button,  Hawaiian  Volcanoes,  4th  An.  Rept.  U.  S.  Geol.  Sur- 
vey, p.  8. 


308  PHYSICAL  GEOGRAPHY 

Heilprin,  Mt.   Pelee  and  the  Tragedy  of  Martinique,   Lippin- 

cott,  Phila.,  1903,  $3. 
Phillips,  Vesuvius,  The  Macmillan  Ck).,  1869. 
Lobley,   Mount  Vesuvius,  London,   1889. 
Hovey,  Eruptions  of  1902,  of  La  Soufriere,   St.  Vincent  and 

Mt.  Pelee,  Amer.  Jour.  Sci.,  Nov.,  1902,  vol.  14,  p.  319. 
Earthquakes : 

Arland,  Great  Earthquakes,  New  York,  1887. 

Button,   Earthquakes  in  the   Light  of  the   New  Seismology, 

London,  1904. 
Charleston  Earthquake,  9th  An.  Rept.  U.  S.  Geol.  Survey. 
Milne,  World-Shaking  Earthquakes,  Scottish  Geog.  Mag.,  Oct., 

1902. 
Perrine,  Earthquakes  in  California,  Bull.  161,  U.  S.  Geol.  Surv. 
San  Francisco  Earthquake,  National  Geog.  Mag.,  May,  1906, 

Pop.  Sci.  Mo.,  Aug.,  1906. 


CHAPTER  IX 
PHYSIOGRAPHIC  FEATURES  OR  FORMS  OF  RELIEF 

Plains,  Plateaus,  Mountains 

If  one  should  travel  westward  across  the  United  States 
from  Atlantic  City  on  the  eastern  coast  to  San  Francisco 
on  the  western,  he  would  traverse  a  succession  of  plains, 
plateaus  and  mountains.  The  same  would  be  true  on  any 
other  continent,  but  the  order,  relative  size  and  features 
of  detail  would  be  different.  The  surface  of  all  the  con- 
tinents and  islands  consists  of  these  three  natural  features 
with  endless  variety  in  each. 

The  line  of  separation  between  mountains  and  plateaus 
or  between  plateaus  and  plains  is  not  always  sharply 
marked.  In  travelling  west  across  the  Appalachian  plateau 
it  would  be  difficult  for  any  one  to  locate  the  exact  spot 
where  he  passes  from  the  plateau  to  the  Mississippi  plains. 

A  desert  is  a  specified  form  of  one  of  the  other  physio- 
graphic features,  a  form  based  on  a  difference  in  climate. 
It  may  be  part  or  all  of  a  plain  or  plateau  and  may  contain 
mountains. 

PLAINS 


i 


246.  Plains  arej  areas  of  low  relief,  comparatively 
smooth  surfaces,  generally  at  no  great  elevation  above  sea 
level.  However,  the  Great  Western  Plains  rise  towards 
the  west  to  heights  several  thousand  feet  above  that  of 
some  plateaus.  Why  are  they  called  plains  instead  of 
plateaus  ? 

The  greater  part  of  the  food  of  the  world  is  raised  on 

309 


310 


PHYSICAL  GEOGRAPHY 


plains,  and  for  that  and  other  reasons  (what  other  reasons 
can  you  give?)  the  greater  part  of  the  population  of  the 
world  lives  on  the  plains.  They  are  therefore  important 
geographic  features  from  an  economic  standpoint. 


Pig.  218.  Banded  coastal  plain.  The  strata  dip  towards  the  sea.  The  hard 
layers  form  ridges  with  steep  face  towards  the  interior.  The  consequent 
streams  flow  towards  the  sea.  The  subsequent  streams  develop  along  the 
edges  of  the  soft  layers  at  right  angles  to  the  first  streams.  Conditions 
are  favorable  for  artesian  wells. 


A  coastal  plain  is  an  uplifted  portion  of  the  continental 
shelf  which  has  been  added  to  the  former  land  area  as  the 
shore  line  receded  seaward.  A  coastal  plain  may  be  nar- 
row and  composed  of  one  kind  of  material,  resulting  in  a 
simple   consequent  drainage   in   which  the   main  streams 


PHYSIOGRAPHIC    FEATURES  311 

flow  in  the  general  direction  of  the  slope  of  the  land ;  or  it 
may  be  broad  and  composed  of  successive  layers  of  differ- 
ent kinds  of  rocks  which,  if  they  differ  in  hardness,  result 
in  parallel  ridges,  and  belts  of  different  kinds  of  soil  par- 
allel to  the  shore.  Such  a  plain  may  be  distinguished  from 
the  preceding  by  calling  it  a  helted  or  handed  coastal  plain. 
Study  carefully  fig.  218  and  explain  the  significance  of  the 
name. 

If  some  of  the  layers  are  more  porous  than  others,  as 
is  generally  the  case,  the  conditions  are  favorable  for 
artesian  wells.  Long  Island,  Southeastern  New  Jersey, 
and  Eastern  Maryland  are  portions  of  a  belted  coastal 
plain  composed  of  beds  of  sand  and  clay  dipping  towards 
and  underneath  the  sea.  The  rain  that  falls  on  the  out- 
cropping edges  of  the  sand  layers  is  carried  down  as 
groundwater  underneath  the  overlying  clay  beds  which 
prevent  its  escape  to  the  surface,  and  hence  causes  accumu- 
lation under  pressure  and  consequent  rise  of  the  water  in 
the  artesian-well  opening.  Artesian  wells  may  be  sunk 
on  the  extension  of  such  a  plain  under  the  shallow  sea  as 
well. as  on  the  land  plain.     (See  figs.  219  and  35.) 

An  ewhayed  coastal  plain  is  one  which,  after  elevation  and 
erosion,  has  been  depressed,  causing  the  shore  line  to  again  ad- 
vance on  the  land.  The  sea  will  now  advance  far  up  the  river 
valleys  forming  bays  and  estuaries,  thus  drowning  the  lower  val~ 
leys  and  dismembering  many  of  the  streams.  In  the  case  of  the 
belted  plains,  there  may  be  one  or  more  lagoons  parallel  with  the 
shore  over  the  eroded  surface  of  the  softer  layers  which  had 
been  worn  down  nearly  to  sea  level  before  the  depression.  The 
higher,  harder  rocks  then  form  peninsulas  or  islands. 

There  are  many  ancient  coastal  plains  now  in  the  interior  of 
the  continents  and  remote  from  the  sea,  because  since  their  first 
uplift  there  have  been  successive  additions  to  the  plain.  In 
some  instances  ev^en  mountain  ranges  have  been  elevated  be- 
tween the  old  coastal  plain  and  the  present  sea  shore. 

Most  of  the   ancient  coastal  plains  that  are  now  far  inland 


312 


PHYSICAL  GEOGRAPHY 


PHYSIOGRAPHIC    FEATURES  313 

are  more  diversified  than  the  recent  ones.  The  rocks  are  gener- 
ally harder  and  more  resistant  and  frequently  the  surface  is 
more  rugged. 

The  greater  part  of  New  York  State  has  been  in  times  past 
part  of  a  coastal  plain.  The  sea  surrounded  the  Adirondack 
Mountains  and  by  successive  uplifts  of  the  land  the  shore-line 
retreated  south  and  west,  exposing  additional  strips  of  coastal 
plain.     Study  a  geological- map  of  New  York. 

Economic  features  of  coastal  plains.  Coastal  plains  are 
generally  covered  with  a  good  soil,  they  have  a  good  water 
supply,  and  commonly  a  fairly  regular  surface,  all  of 
which  favor  an  extensive  agricultural  industry.  Road 
construction  is  for  the  most  part  easier  than  in  the  hill 
country,  which  fact  favors  both  agriculture  and  commerce. 
Sometimes  the  commerce  by  water  is  hampered  by  the  lack 
of  good  harbor  facilities,  but  generally  there  is  consider- 
able water  traffic  both  in  the  coast  trade  and  on  the  rivers 
which  flow  across  the  plain.  They  also  favor  manufactur- 
ing industries  where  there  are  good  harbors  or  navigable 
streams,  because  of  the  favorable  position  for  distributing 
the  products  of  the  factory. 

247.  Alluvial  Plains.— Flood  plains  occur  along  the 
older  portions  of  nearly  all  rivers.  As  soon  as  the  river 
has  cut  down  to  grade,  it  begins  cutting  at  the  cliffs,  some- 
times on  one  side  and  sometimes  on  the  other.  Part  of 
this  material  and  part  of  that  brought  from  the  head 
waters  is  spread  out  over  the  floor  of  the  valley,  building 
up  a  flood  plain  over  which  the  river  takes  a  meandering 
course.  During  high  water  when  the  river  overflows  its 
banks,  sediment  is  deposited  over  all  the  area  flooded, 
but  in  greater  quantities  along  the  immediate  banks  of  the 
stream,  building  up  embankments  or  natural  levees,  which 
grow  higher  from  flood  to  flood  until  finally  the  river 
breaks  through  and  takes  a  new  course  on  the  lower  part 


314 


PHYSICAL  GEOGRAPHY 


of  the  plain  which  in  turn  is  built  up  in  a  similar  manner. 
(See  figs.  220,  221,  54,  55,  58  and  67.) 

One  effect  of  the  upbuilding  of  the  natural  levee  is  to  cause 
the  surface  of  the  plain  to  slope  away  from  rather  than  towards 


Fxu.    220.      View  on    the   flood   plain   of   the   Bittcnoot    river   in    Montana.      The 
alluvial  soil   is  very  productive.      (U.   S.   Geol.  Survey.) 


the  river  channel,  so  much  so  that  in  some  places  small  streams 
develop  on  the  levee  bank  and  flow  directly  away  from  the  chan- 
nel into  the  river  swamp.  (See  Donaldsonville  topographic  sheet.) 
Furthermore,  the  levee  forms  an  obstruction  to  the  junction  of  a 
tributary  with  the  main  stream.  On  the  lower  Mississippi  river 
flood  plain,  some  of  the  tributaries  flow  along  the  outer  margin 
of  it  for  many  miles  before  they  flnd  an  opening  through  the 
levee  into  the  main  river.     (See  fig.  221.) 

On  the  delta,  where  there  are  no  bordering  bluffs,  portions 
of  the  main  stream  break  through  or  overflow  the  levee  and  flow 
directly  to  the  sea  as  distributaries. 

The  delta  plain  forms  the  continuation  seaward  of  the  river 


PHYSIOGRAPHIC    FEATURES 


315 


flood  plain.     The  delta  plain  of  the  Nile  has  for  many  centuries 
been  one  of  the  most  populous  portions  of  the  globe. 

The  greater  part 
of  a  flood  plain  is 
fertile  land,  well 
adapted  to  agricul- 
ture because:  (1)  It 
is  covered  with  rich 
humus  carried  from 
the  uplands  and  hill- 
sides; (2)  the  fertil- 
ity is  renewed  from 
time  to  time  by  the 
overflows,  which  de- 
posit a  new  layer  of 
fertile  soil;  (3)  the 
water-table  lies  so 
near  the  surface  that 
the  area  does  not  suf- 
fer from  drouth  so 
much  as  the  upland; 
and  (4)  transporta- 
tion and  communica- 
tion, save  in  the  flood 
season,  are  easy  by 
road,  railway  and  ^ 
river.  For  these  rea- 
sons a  large  part  of 
the  food  products  of 
the  world  is  raised 
on  alluvial  river 
plains  and  deltas. 


Fig.  221.  Map  of  a  portion  of  the  lower  Miss- 
issippi River  flood  plain  showing  effect  on 
the  tributaries  of  the  up-building  of  the  flood 
plain  along  the  channel.  Note  how  many  of 
the  tributaries  run  nearly  parallel  with  the 
river  for  long  distances.  The  dark  lines  along 
the  river  are  artificial  levees.  (U.  S.  Geol. 
Survey. ) 


The  greatest  drawbacks  to  prosperity  on  the  flood  plains 
are   (1)   destruction  of  life  and  property  from  the  floods, 


316  PHYSICAL  GEOGRAPHY 

and  (2)  malarial  climate  from  the  many  mosquitoes  which 
breed  in  the  stagnant  water.  Many  attempts  have  been 
made  to  remedy  the  danger  from  floods  by  building  higher 
embankments  on  the  levees  to  keep  the  water  in  the  chan- 
nel. But  the  exceptionally  high  flood  finally  overflows 
and  causes  injury  to  both  life  and  property.     In  a  dry 


Fig.  222.  Brazos  Island,  opposite  Point  Isabel,  near  Brownsville,  Texas. 
The  effect  of  grass  in  holding  the  drifting  sand.  River  flood  plain  in  an 
arid  region.     (W.  L.  Bray.) 

climate,  a  shallow  river  sometimes  deposits  much  sand  in 
and  along  the  channel.  Strong  winds  spread  the  sand 
over  the  adjoining  fertile  areas.     (See  fig.  222.) 

248.  Lacustrine  Plains.— A  lacustrine  plain  is  one 
that  was  formerly  covered  with  a  lake.  The  plain  may 
represent  the  entire  area  of  the  former  lake  or  only  a  por- 
tion of  it  and  may  be  produced  by:  (1)  the  filling  of  the 
lake;   (2)  the  draining  of  it  by  the  cutting  down  of  the 


PHYSIOGRAPHIC    FEATURES  317 

outlet;  (3)  the  evaporation  of  the  water  by  a  change  in 
climate;  (4)  diastrophism  elevating  a  portion  or  all  of 
the  lake  bottom;  or  sometimes  a  combination  of  two  or 
more  of  the  above  ways. 

The  famous  celery  and  onion  black  land  areas  over  the  north- 
ern United  States  are  almost  all  lake  plains  formed  by  the  filling 
partially  or  completely  of  small  lake  basins  with  vegetable  and 
animal  matter.  The  numerous  vlies  or  meadows  of  the  Adiron- 
dacks  are  similarly  formed. 

The  great  wheat  lands  of  the  northwest,  in  Dakota,  Montana 
and  Canada  are  lake  plains  on  the  bottom  of  large  extinct  lakes. 
Lacustrine  plains  are  almost  universally  fertile,  since  they  con- 
tain so  much  organic  matter  and  the  rock  material  is  so  very 
finely  disintegrated. 

In  places  around  the  margin  of  extinct  or  fossil  lakes,  there 
are  sand  and  gravel  deposits  corresponding  to  the  sand  and 
gravel  beaches  now  forming  in  many  places  along  the  shores  of 
the  larger  lakes.  These  are  lacking  in  fertility  and  have  little 
value  for  agriculture,  but  in  the  vicinity  of  cities  they  furnish 
sand  and  gravel  for  building  purposes.  Nearly  all  the  sand  and 
gravel  used  in  Syracuse,  New  York,  are  obtained  from  the  old 
beaches  of  an  extinct  lake  on  the  bed  of  which  a  considerable 
portion  of  the  city  is  built. 

249.  Salt  and  Alkali  Plains.— Lacustrine  plains  in  arid 
regions  are  sometimes  covered  in  part  at  least  with  salt  or 
alkalies,  sometimes  in  such  quantities  that  they  can  be 
shovelled  into  cars  and  shipped  to  market.  Shipping  salt 
in  this  way  was  an  important  industry  at  Salton,  Cali- 
fornia, before  the  Colorado  River  broke  loose  and  covered 
the  area  with  water.  Travel  over  the  extensive  alkali  plains 
west  of  Great  Salt  Lake  in  Utah  and  Nevada  is  disagree- 
able in  dry  weather  because  of  the  alkali  dust. 

250.  Glacial  Plains. — A  continental  glacier,  such  as 
that  which  covered  the  northern  United  States  in  former 
times,  leaves  many  plains,  large  and  small,  on  the  area 
which  it  covered.     Probably  the  glacier  does  not  produce 


318  PHYSICAL  GEOGRAPHY 

many  plains,  but  it  modifies  the  surface  of  those  already  in 
existence,  possibly  increasing  the  size  of  some  of  them. 

The  glacial  plains  may  be  covered  with  glacial  till  or  boulder- 
clay  or  with  sand  and  gravel  as  in  the  overwash  plains  and 
glacial  aprons.  In  some  places  they  are  partly  covered  with 
large  boulders.  Many  of  the  lacustrine  plains  of  the  northern 
United  States  are  indirectly  glacial,  as  it  was  the  glacier  that 
formed  the  lake  basin  and  sometimes  helped  to  fill  it. 

251.  Peneplains.— Some  plains  are  formed  by  exten- 
sive erosion  of  a  former  plateau  or  mountain  area.  As  the 
rivers  with  their  tributaries  cut  the  upland  plateau  or 
mountain  down  nearly  to  base  level,  the  divides  between 
the  valleys  are  finally  lowered  to  near  the  valley  levels 
until  the  entire  area  approaches  a  plain  surface  which  is 
called  a  peneplain,  meaning  almost  a  plain.  The  harder, 
more  resistant  rocks  on  the  area  remain  as  hills  or  eleva- 
tions on  the  peneplain,  and  the  more  prominent  ones  are 
known  as  monadnocks. 

Nearly  all  of  New  England  was  at  one  time  reduced  by  ero- 
sion to  a  peneplain,  which  was  then  elevated.  The  streams 
deepened  and  widened  their  valleys  until  the  uplifted  plain  or 
plateau  was  much  dissected.  The  upland  areas  between  the 
streams  are  all  that  is  left  of  the  former  peneplain.  (See  "The 
New  England  Plateau"  by  W.  M.  Davis,  one  of  the  National 
Geographic  Monographs.) 

Around  the  base  of  high  mountains  there  are  fre- 
quently plains,  large  and  small,  that  have  been  built  up  by 
the  material  carried  down  the  mountain  and  spread  on  the 
low  ground  at  the  base.  They  are  conspicuous  topographic 
features  in  the  arid  and  semi-arid  mountainous  regions  of 
the  west  and  southwest  United  States.  Fig.  223  is  a  view 
on  one  of  these  intra-montane  (among  the  mountains) 
plains  near  Acton  in  southern  California.  Such  areas 
have  been  called  plains  of  aerial  aggradation. 


320  PHYSICAL    GEOGRAPHY 

252.  Prairies.— The  prairies  or  treeless  plains  are 
nearly  everywhere  covered  with  a  fertile  soil,  and  in  a 
moist  climate  in  the  wild  state  they  are  covered  with  a 
dense  growth  of  prairie  wild  grass  which  formerly  sup- 


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Fiu.  224.  View  on  the  Great  Western  Plains.  The  plains  are  mostly  treeless 
and  have  only  a  light  rainfall.  The  chief  industry  is  grazing  herds  of 
cattle,  sheep,  and  horses.  Tilling  the  soil  is  profitable  where  the  surface 
is  irrigated.     The  depression  is  a  buffalo  wallow.      (U.  S.  Gteol.  Survey.) 


ported  great  numbers  of  buffaloes,  antelopes,  wild  horses, 
and  other  animals.  Much  of  the  prairie  land  of  central 
North  America  is  now  under  cultivation  and  includes  part 
of  the  famous  corn  belt  of  the  central  region  and  the  wheat 
belt  of  the  north  and  northwest. 

In  the  semi-arid  region  of  the  middle  west,  the  area  just  east 
of  the  Rocky  Mountains,  the  rainfall  is  not  sufficient  to  grow 
farm  products  without  irrigation,  but  enough  to  produce  a  scanty 
growth  of  a  short  but  very  nutritious  grass,  which  formerly  sup- 
ported vast  herds  of  buffalo  and  which  now  furnishes  food  for 


PHYSIOGRAPHIC   FEATURES  321 

the  great  herds  of  cattle,  sheep,  and  horses  grazing  on  this  area 
which  extends  from  the  Gulf  of  Mexico  entirely  across  the  United 
States  far  into  Canadian  territory. 

Somewhat  similar  plains  occur  in  the  interior  of  other  con- 
tinents. Such  are  the  great  wheat  lands  and  grazing  plains  of 
Argentina  in  South  America,  and  the  great  steppes  or  treeless 
plains  of  Russia  and  southern  Siberia. 

253.  Tundras.—  In  the  far  north  in  both  America  and  the 
Eurasian  continent  are  vast  stretches  of  treeless  plains  covered 
with  a  dense  growth  of  mosses  and  other  plants  in  the  short  sum- 
mer season.  A  few  feet  below  this  growing  vegetation  and  con- 
tinuing for  depths  sometimes  of  a  hundred  feet  or  more,  the 
ground  is  perpetually  frozen.  These  frozen  plains  are  known  as 
tundras. 

PLATEAUS 

254.  Many  upland  plains  are  called  plateaus.  What 
is  it  that  distinguishes  a  plateau  from  a  plain?  It  is  not 
height  alone,  for  the  area  of  the  Great  Western  Plains  east 
of  the  Rocky  Mountains  is  higher  than  either  the  Ozark  or 
the  Alleghany  plateau. 

In  passing  across  the  Great  Western  Plains  in  any 
direction  there  is  nothing  to  indicate  their  elevation,  noth- 
ing that  appeals  to  the  senses.  The  higher  portions  are 
nowhere  bordered  by  land  at  a  perceptibly  lower  level. 
Even  the  large  rivers  flow  on  the  top  of  the  plain.  The 
Rocky  Mountains  and  the  Black  Hills  stand  high  above 
them.  Despite  the  fact  that  on  the  plains  east  of  the 
Rocky  Mountains  the  land  is  several  thousand  feet  above 
sea  level,  it  is  much  lower  than  the  mountains  which  border 
them. 

As  one  crosses  the  eastern  margin  of  the  Alleghany 
plateau,  he  finds  an  abrupt  descent  to  the  deep  valleys 
separating  it  from  the  Alleghany  ridges.  The  rivers  which 
flow  across  the  plateau  flow  in  deep  valleys,  and  hence  by 
comparison  the  land  appears  to  be  high.     A  plateau  is  an 

21 


322 


PHYSICAL  GEOGRAPHY 


inland  plain  bordered  in  part  at  least  by  a  perceptibly 
lower  area,  that  causes  the  plateau  to  appear  higher  by 
comparison. 


Fia.  225.       View    in   Ausable   chasm,    New   York.      A   deep   chasm    cut   l>y   the 
Ausable  river  in  a  hard  sandstone  rock,      (H,  W.  Brock.) 

255.  Canyons. — Plateaus  have  one  characteristic  topo- 
graphic feature  not  common  to  plains,  namely,  deep  nar- 
row valleys  called  canyons  in  the  western  United  States 
and  gorges,  chasms  and  glens  in  the  east.  The  Colorado 
Canyon  in  Arizona,  the  Niagara  gorge,  Ausable  Chasm  and 
Watkins  Glen  in  New  York  are  alike  in  being  narrow  and 
deep. 

The  western  canyons  are  generally  deeper  than  the  eastern 
gorges  because  the  plateaus  are  higher,  and  hence  there  is  a 
greater  thickness  of  rocks  through  which  the  stream  can  cut  be- 
fore reaching  its  grade.  Yet  there  are  many  small  canyons  in 
the  west  on  tributaries  of  the  great  rivers  that  are  not  as  deep 
as  Niagara  Gorge  or  Watkins  Glen.     (See  figs.  225,  226  and  97.) 

The   western  canyons  are   generally  narrower   in   proportion 


PHYSIOGRAPHIC   FEATURES  323 

to  the  depth  than  the  eastern  valleys,  because  (1)  the  arid 
climate  is  less  favorable  to  the  action  of  frost  and  other  weather- 
ing agencies  which  wear  away  the  cliffs  and  widen  the  valley  in 
the  moist  climate.     (2)  The  absence  of  vegetation,  due  to  aridity. 


Fig.  226.  Entrance  to  Grand  Canyon  of  the  N,  Platte  in  Wyoming.  The 
canyon  has  a  depth  of  1000  feet  in  places  in  hard  sandstones,  limestones, 
and  granite. 

and  the  concentration  of  the  rainfall  in  a  few  heavy  showers, 
favors  corrasion  in  the  stream  channel,  thus  deepening  the  valley 
at  the  expense  of  weathering  on  the  sides.  (3)  The  western 
area  has  been  elevated  higher  and  more  rapidly. 

Tributary  canyons  develop  in  time,  and  as  they  increase  in 
number,  length  and  size,  the  plateau  is  dissected  into  first  table- 
lands, commonly  called  mesas  in  the  west.  The  table  lands  are 
further  dissected  or  worn  down  to  smaller  remnants  forming 
hills  or  peaks,  called  buttes  in  the  west.  Name  the  necessary 
conditions  for  the  formation  of  canyons.     (Figs.  227  and  228.) 

256.  Faults.— The  rock  layers  of  a  plateau  may  be 
fractured  or  broken,  the  plane  of  fracture  being  a  fissure 
or  crack.     It  frequently  happens  that  the  rocks  on  one  side 


324 


PHYSICAL  GEOGRAPHY 


Fia.  227.  View  west  of  Sheep  Mountain,  S.  Dakota.  Mesa  in  background. 
Cactus  flats  in  the  foreground.  How  does  the  vegetation  indicate  dry 
climate?  What  indication  is  there  of  considerable  rainfall?  (U.  G. 
Cornell.) 


Fio.  228.  Alkali  Buttes,  Weston  Co.,  Wyoming. — Mesa  in  the  far  background. 
The  mesa  at  one  time  extended  over  the  entire  area.  The  buttes  are 
remnants  of  the  former  extension.     (U.  S.  Geol.  Survey.) 


PHYSIOGRAPHIC   FEATURES 


325 


of  a  fissure  are  elevated  more  than  those  on  the  other  side. 
Such  a  displacement  of  the  rocks  along  a  fissure  is  called 
a  fault,  and  the  plane  of  the  fissure  becomes  a  fault  plane. 
The  direction  of  this  fault  plane  on  the  surface  is  the 
fault  line.  The  projection  of  the  uplifted  portion  above 
the  other  side  is  called  the  fault  scarp.  Where  the  rocks 
are  much  broken  and  the  fault  scarps  are  numerous,  the 
surface  of  the  plateau  may  be  made  very  irregular.  The 
Block  Mountains  are  supposed  to  be  formed  in  this  way 
by  the  fracturing  of  the  plateau  and  tilting  of  the  great 
earth  blocks  along  the  fault  planes. 


1  IT 

Fig.   229.      Diagram    of    fault.      AB,    fault    plane.      AC,    fault    scarp    which    is 
worn  back  to  DC   in  11.      1  is  a  normal  fault.      11  a  reverse  fault. 


Faults  occur  in  mountains  and  plains  as  well  as  on  the 
plateaus,  but  in  many  cases  they  are  not  noticeable  on  the  surface, 
as  after  the  elevation  of  the  fault  scarp  the  eroding  agencies 
wear  down  the  uplifted  portion  until  the  two  sides  are  again  on 
the  same  level.  A  fault  line  is  sometimes  indicated  on  the  sur- 
face by  a  succession  of  springs  which  emerge  along  the  line  of 
displacement.     (Figs.  229  and  230.) 

257.  Economic  Importance  of  Plateaus.— The  plateaus 
are  generally  not  so  productive  and  populous  as  plains. 
First,  they  are  likely  to  suffer  more  for  want  of  water,  as 
the  water  plane  is  not  so  near  the  surface.  Second,  the 
soil  is  frequently  not  so  fertile  as  in  the  valley  or  on  the 


326 


PHYSICAL  GEOGRAPHY 


low  plain.  Third,  transportation  is  generally  more  ex- 
pensive than  on  the  plains,  due  to  distance  from  the  sea 
coast,  absence  of  navigable  rivers,  and  the  cost  of  bridging 
the  numerous  canyons. 


Fia.  230.  Fault  at  Jamesville,  N.  Y.  Locate  the  fault  plane  and  determine 
from  the  way  the  rocks  are  bent  and  broken  whether  the  rocks  on  the  left 
moved  up  as  in  229  11  or  down  as  in  229  1. 


The  climate  is  generally  more  healthful  on  the  plateau 
than  on  the  low  plains,  as  the  air  is  lighter,  purer,  and 
freer  from  malaria.  The  arid  plateaus  have  and  probably 
always  will  have  a  scanty  population,  because  of  the  dif- 
ficulty of  getting  water.     In  a  moist  climate  the  plateau 


PHYSIOGEAPHIC    FEATURES 


327 


may  have  a  prosperous  farming  population,  which,  how- 
ever, is  generally  less  dense  than  on  the  low  plains. 


Fig.  231.  Entrance  to  coal  mine  in  the  Alleghany  Plateau,  Allegany  County, 
Md.  In  a  plain  country  like  Illinois  coal  is  lifted  to  the  surface  through 
vertical  shafts.  Here  the  coal  is  drawn  to  the  surface  on  cars  on  a  nearly 
level  roadway,  extending  from  the  sides  of  the  deep  valleys  in  under  the 
plateau.      (See  also  Figs  69  and  185)    (Md.  Geol.  Survey.) 


The  Alleghany  plateau  through  Pennsylvania  and  southward 
is  underlain  by  numerous  beds  of  coal  and  fire-clay.  Many  deep 
valleys  cut  through  these  beds,  and  expose  them  to  view  on  the 
hillsides  in  favorable  position  for  mining.  (See  fig.  231,  also 
fig.  69.) 

m  nearly  all  plateaus,  beds  of  sandstone,  limestone  and  other 
rocks  suitable  for  building  purposes  are  exposed  on  the  steep 
slopes  bordering  the  valleys,  where  many  quarries  have  been 
opened  for  the  purpose  of  obtaining  the  stone. 


328  PHYSICAL  GEOGRAPHY 

DESERTS 

258.  The  desert  is  such  a  striking  contrast  to  the  grass- 
and-forest-covered  plains  and  plateaus  of  the  humid  areas 
as  to  seem  like  a  new  world.  Frequently  the  first  impres- 
sion on  visiting  a  desert  is  that  one  is  in  dreamland  where 
things  are  not  what  they  seem.     Lakes  appear  where  there 


Pig.  232.     On   the   Mojave  desert   near  Bagdad,   California.      There   is  a  great 
scarcity  but  not  absence  of  life  and  water.     (Detroit  Publishing  Co.) 

is  no  water  and  sometimes  seem  to  run  over  the  moun- 
tains into  the  sky.  Rocks,  hills  and  mountains  appear  dis- 
torted and  unreal.  Grave  and  sombre  tints  have  replaced 
the  green  leaves  and  bright  flowers.  One  seems  to  be  look- 
ing through  a  yellow  or  brown  glass. 

One  of  the  most  impressive  features  of  the  desert  is 
the  absence  of  noise,  the  apparent  absolute  stillness.  One 
is  surprised  to  hear  the  ticking  of  his  watch  in  his  pocket, 


PHYSIOGRAPHIC   FEATURES  329 

the  beating  of  his  heart,  and  other  sounds  which  in  our 
wave-disturbed  atmosphere  are  never  heard.     (Fig.  232.) 

Sombre,  dreary  and  desolate  as  the  desert  may  appear 
at  first  sight,  its  solitude  has  charms  that  often  prove  ir- 
resistible. No  peoples  are  so  wedded  to  their  native  soil 
as  the  nomads  of  the  desert.  Rarely  indeed  do  they  emi- 
grate either  singly  or  collectively. 

Definition.  If  it  is  the  absence  or  scarcity  of  life  due 
to  unfavorable  conditions  that  makes  a  desert,  then  there 
are  three  classes  of  deserts : 

(1)  The  dry  desert,  barren  for  want  of  sufficient  rain; 
(2)  the  cold  desert^  barren  because  of  the  low  temperature 
and  excess  of  snow  which  occurs  in  polar  regions  and  high 
mountains;  (3)  the  wet  desert,  in  mid  ocean,  where  the 
barrenness  is  caused  by  the  darkness,  cold,  and  pressure. 
The  surface  of  the  ocean  teems  with  life,  and  there  is  some 
life  in  the  bottom  of  the  deep  sea,  but  the  great  body  of 
the  ocean  included  between  these  two  zones  is  almost  barren 
of  life  and  in  that  respect  it  is  a  great  desert. 

The  dry  deserts,  although  smaller  than  the  others,  are 
the  ones  commonly  meant  when  the  word  is  used ;  that  is, 
in  addition  to  the  idea  of  barrenness  or  scarcity  of  life, 
the  word  desert  conveys  frequently  the  idea  of  aridity  or 
scarcity  of  water.  Used  in  this  sense,  the  second  and  third 
classes  of  deserts  mentioned  disappear. 

In  the  broader  sense,  a  desert  is  a  region  conspicuous 
for  the  scarcity  or  absence  of  life,  especially  vegetable  life. 
In  a  more  limited  sense,  the  barrenness  is  due  to  scarcity 
of  moisture. 

259.  Dry  Deserts.— The  definition  sometimes  given 
for  a  desert,  that  it  is  a  rainless  region,  is  a  faulty  one  as 
no  region  is  entirely  rainless  and  generally  there  is  no 
sharp  line  of  separation  between  a  desert  and  a  semi-arid 
region.     Ordinarily  less  than  20   inches   annual  rainfall 


330 


PHYSICAL  GEOGRAPHY 


means  a  region  too  dry  to  be  cultivated  and  hence  classed 
as  semi-arid  and  used  for  grazing  purposes,  unless  it  can  be 
irrigated.     But  other  factors  than  annual  rainfall  must  be 


FiO.  233.  Big  Bad  Lands,  S.  D.  Erosion  by  heavy  rains  on  soft  material. 
The  annual  rainfall  is  light  but  concentrated.  The  region  is  desert  be- 
cause of  the  irregular  distribution  of  the  rainfall.      (U.    S.   Geol.   Survey.) 

considered.  Much  depends  on  the  distribution  of  the  rain, 
whether  it  falls  in  the  growing  season  or  not,  whether  it  all 
falls  in  one  or  two  heavy  downpours,  or  is  distributed 
through  the  year.     (Fig.  233.) 

The  rains  of  the  desert  are  generally  violent,  often  accom- 
panied by  cloud  bursts.  The  effect  produced  on  the  surface  is 
to  form  raging  torrents  which  erode  deep  gullies  or  channel-ways 
known  as  wadies  in  the  Sahara,  and  as  arroyos  or  barrancas  in 
the  desert  areas  of  the  southwest  United  States.  (See  sec.  94.) 
These  watercourses,  strewn  with  boulders,  sand  and  driftwood, 
are  characteristic  features  of  aridity.     (Figs.  234  and  72a.) 

Wind  is  an  important  sculpturing  and  transporting  agent  in  a 
desert  region.  Hot  and  dry  atmosphere  produces  a  dry  sur- 
face to  the  soil.  The  absence  or  scarcity  of  vegetation  causes 
a  bare  surface  susceptible  to  the  action  of  the  winds  which  blow 


PHYSIOGRAPHIC    FEATURES  331 

the  dust  and  sand  from  place  to  place.  The  accumulation  of  the 
sand  forms  dunes  not  unlike  those  in  moist  climates.  (See  sec- 
tion 185.) 

The  fine  sand  and  dust  carried  by  the  winds  against  or 
across  the  surface  of  the  bare  rocks  acts  as  a  sand  blast  in 
grinding  and  wearing  the  surface,  even  the  hardest  of  rock  sur- 


FlG.  234.     Arroyo  or  stream  channel   in  Arizona.      The   channel   is   dry  nearly 
all  the  time,  but  occasionally  it  is  swept  by  a  torrent.      (D.  T.  McDougal.) 

faces,  (Fig.  120a  shows  some  quartz  pebbles,  among  the  hardest 
of  common  rocks,  that  were  worn  by  the  desert  sand  blast.  Com- 
pare them  with  the  stream  pebbles  and  the  glacial  pebbles.) 

The  hard  granite  rocks  of  the  desert  are  frequently  worn  In- 
to weird  shapes  by  the  wind-blown  sands.     (See  fig.  191.) 

260.  Desert  Life.— The  life  of  the  desert  is  in  strong 
contrast  to  that  in  a  humid  area.  One  of  the  most  prom- 
inent characteristics  of  the  life  is  its  scarcity,  but  not  often 


332 


PHYSICAL  GEOGRAPHY 


is  it  wholly  absent.     Sombre  colors  prevail  in  both  animal 
and  vegetable  forms. 

"The  life  on  the  desert  is  peculiarly  savage.    It  is  a  show  of 
teeth  in  bush  and  beast  and  reptile.    At  every  turn  one  feels  the 


gill 

m                      jLfiaBp|Mjyl  1     li  HUy      ' 

^"    1P|BS^ 

Fig.  235.     Uesert  vegetation.     Cacti  and  mesquite  near  Torres, 
(See  also  Fig.  227.)    (D.  T.  McDougal.) 


Sonora,  Mex. 


presence  of  the  barb  and  thorn,  the  jaw  and  paw,  the  beak  and 
talon,  the  sting  and  the  poison  thereof."  (The  Desert,  Van  Dyke, 
Page  27.) 

The  more  common  plants  of  the  American  desert  are  the  sage 
brush,  greasewood,  cactus,  yucca,  bunch  grass  and  mesquite. 

The  animals  are  the  coyote,  jack  rabbit,  antelope,  prairie 
dog,  rattlesnake,  horned  toad  and  Gila  monster.  (See  figs.  235, 
236,  237  and  227.) 

261.  Distribution  of  Deserts.— Probably  the  largest 
of  all  the  dry  deserts  is  the  Sahara  of  Africa.  Extensive 
desert  areas  occur  in  central  Asia,  and  in  central  and  west- 
em  Australia.  There  is  a  narrow  strip  of  desert  on  the 
west  coast  of  South  America,  a  region  that  is  probably  as 
nearly  rainless  as  any  on  the  globe. 


PHYSIOGRAPHIC    FEATURES  333 

There  are  several  desert  areas  in  the  west  and  south- 
west United  States,  areas  of  considerable  extent  but  grow- 
ing smaller  each  year.     In  the  older  books  the  Great  West- 


I'lG.  236.  Sage  brush  (Artemisia  tridentata),  the  most  common  plant  of  tho 
arid  plains  of  Western  United  States.  View  near  Elko,  Nevada.  Snow- 
capped Ruby  Mountains  in  the  distance.      (D.   T.    McDougal.) 

ern  Plains  between  the  Missouri  river  and  the  Rocky  Moun- 
tains were  called  the  ''Great  American  Desert."  The  area 
now  supports  a  large  and  increasing  population.  The  title 
was  next  applied  to  the  area  west  of  the  mountains,  includ- 
ing at  first  all  that  vast  area  between  the  Rocky  Mountains 
and  the  Sierra  Nevadas ;  but  as  it  became  better  known  the 
desert  portion  of  this  area  decreased  so  rapidly  that  it  does 


334 


PHYSICAL  GEOGRAPHY 


not  now  appear  on  the  United  States  map  at  all.  There 
are  several  desert  areas  of  appreciable  size  in  Utah,  Nevada, 
New  Mexico,  Arizona,  and  southern  California,  but  they 
are  separated  by  productive  or  partially  productive  areas 
which  are  gradually  increasing  in  size  at  the  expense  of 
the  barren  ones. 


Fig.  237.  Knight's  Temple  in  Bates  Hole,  Wyoming.  Life  in  a  region  of 
slight  but  concentrated  rainfall.  Trees  along  the  creek,  scanty  grass  in 
foreground.  The  hills  in  the  background  are  nearly  devoid  of  plant  life. 
(U.  G.  Cornell.) 


The  desert  areas  have  been  greatly  diminished  by  irrigation, 
and  other  portions  have  been  brought  under  cultivation  by  tilling 
the  soil.  The  process  known  as  dry  farming  is  now  carried  on 
extensively  in  areas  formerly  considered  barren.  In  these  ways 
a  thrifty  and  industrious  population  is  gradually  encroaching  up- 
on and  thus  diminishing  the  desert  areas  of  the  western  United 
States.  The  relatively  small  portions  of  the  great  desert  areas 
that   have   not  been   thus  brought  under  subjection  have   been 


PHYSIOGRAPHIC    FEATURES  335 

robbed  of  their  former  terrors  by  the  horse,  the  steam  and  elec- 
tric railways  and  the  automobile. 

MOUNTAINS 

262.  Mountains  are  the  most  conspicuous  features  of 
the  earth's  physiography.  We  can  see  part  of  the  ocean 
or  part  of  a  large  plain,  but  we  cannot  see  as  much  of 
either  as  we  can  of  a  mountain.  Hence  the  mountains 
impress  us  with  ideas  of  vastness,  sublimity,  and  Omni- 
potent power. 

Long  ago  man,  in  his  imagination,  peopled  the  moun- 
tains with  giants,  goblins,  and  dragons  so  that  they  were 
objects  of  dread  and  were  avoided  by  all.  A  mountain 
range  was  then  a  practically  impassable  barrier.  But  in 
the  18th  century  the  ghosts  and  goblins  that  had  been  hov- 
ering in  the  shadows  of  superstition  began  to  disappear  be- 
fore the  bright  headlight  of  scientific  discovery  and  inves- 
tigation and  the  mountains  became  objects  of  attraction 
and  study  rather  than  of  distrust.  People  are  attracted 
to  the  mountains  in  different  ways:  some  for  the  scenery, 
as  nowhere  else  do  we  get  such  grand  and  inspiring  pan- 
oramas; others  for  the  exhilarating  atmosphere  and  the 
fresh,  sparkling  waters;  others  for  the  mineral  wealth; 
others  for  the  timber ;  others  for  the  fish  and  game  products. 

While  mountains  are  interesting  features  to  all,  they  are 
especially  so  to  the  geologist  and  the  geographer  because  they 
whisper  to  him  many  of  the  secrets  of  nature  that  on  the  plains 
are  concealed  beneath  the  heavy  cloak  of  mantle  rock.  In  the 
mountains  this  mantle  is  rent  and  torn  in  many  places,  revealing 
the  history  of  the  past  in  the  structure  of  the  underlying  rocks. 
The  geologist  also  observes  that  while  the  mountains  are  now 
the  highest  portions  of  the  earth,  they  were  born  in  the  ocean 
and  are  truly  children  of  the  sea. 

There  is  no  well-defined  line  of  separation  between  mountains 
and   hills   or  between   mountains    and   plateaus.     Mountains   are 


336 


PHYSICAL  GEOGRAPHY 


higher  and  larger  than  hills,  but  the  elevations  are  comparative. 
Thus  the  Fourche  Mountains  at  Little  Rock,  Arkansas,  are  less 
than  400  feet  above  sea  level,  but  they  are  much  higher  than  any 
other  area  between  them  and  the  Gulf.  The  Blaclc  Hills  in  South 
Dakota  are  many  times  higher  than  the  Fourche  Mountains, 
much  higher  even  than  the  Alleghany  Mountains,  but  they  are 
dwarfed  by  the  lofty  Rocky  Mountains  to  the  west  and  are  pop- 
ularly called  hills. 


Pia.  238.     An  anticline,   Hancock,    Md.  formed  by  the  upward   bending  of 
the  strata.      (U.  S.  Geol.  Survey.) 

Mountains  may  be  caused  by  diastrophism,  folding, 
faulting,  uplifting  with  erosion,  or  volcanic  action.  The 
uplift  accompanied  by  folding  is  called  orogenic  to  distin- 
guish it  from  an  uplift  without  folding,  epeirogenic.  The 
former  produces  mountain  ranges,  the  latter  plateaus  which 


PHYSIOGRAPHIC    FEATURES 


337 


are  dissected  by  streams  into  mountains.     Volcanic  erup- 
tions also  build  up  mountains.      (See  Volcanoes.) 

263.     Folded  Mountains.— Most  of  the  great  mountains 
are  caused  by  orogenic  movements.     The  foldings  are  fre- 


FiG.  239.      Syncline,     three    miles    west    of    Hancock,     Md.      A    ti*ough- shaped 
downward  bending  of  the   strata.      (U.  S.  Geol.  Survey.) 


quently  complex  and  after  the  surface  has  been  subject  to 
erosion  for  a  long  time,  it  is  much  diversified  by  the  more 
rapid  cutting  away  of  the  soft  layers,  which  causes  the 
hard  layers  to  stand  above  the  surface  as  mountain  ridges 
and  hills. 

The  crumpling  of  the  layers  produces  anticline,  syn- 
cline and  monocline  folds.  The  upward  bending  forms 
the  anticline,  the  downward  bending  or  trough  forms  the 
syncline,  while  a  single  bending  from  one  level  to  another 

22 


338  PHYSICAL  GEOGRAPHY 

is  a  monocline.     ( Study  the  diagrams,  figs  240  and  242,  and 
figs.  238  and  239.) 

After  the  bending  of  the  rocks,  the  tops  of  the  anticlines 
form  the  tops  of  the  mountains  or  ridges  where  erosion  begins 
most  actively,  because  they  are  the  higher  points,  and  possibly 
also  because  the  rocks  there  are  more  shattered  and  broken. 
Thus  valleys  form  along  the  anticlines  which  are  cut  down  more 
rapidly  than  the  first  valleys  in  the  synclinal  troughs,  until  in 
time  the  synclines  (the  first  valleys)  are  left  as  mountains  high 
above  the  level  of  the  eroded  anticline  which  now  forms  the  val- 
ley. The  accompanying  diagrams  indicate  how  erosion  has  cut 
off  the  tops  of  the  anticlines  until  the  synclines  stand  at  higher 
levels  and  form  mountains.  Many  of  the  present  ridges  of  the 
Alleghany  and  other  mountains  are  synclines.  When  first  up- 
lifted the  mountain  ridges  were  on  the  anticlines.     (Fig,  240.) 


Fia.  240.  Diagram  illustrating  crumpling  of  the  strata  in  mountain-making. 
A,  anticline ;  S,  syncline ;  M,  monocline.  Dotted  lines  represent  portions 
eroded  after  the  crumpling. 

Parallel  ridges  and  terraced  mountains.  During  the 
erosion  of  the  top  of  a  large  anticline  which  contains  alter- 
nating layers  of  hard  and  soft  rocks,  parallel  valleys  de- 
velop on  the  softer  layers  which  leaves  the  harder  layers 
standing  up  as  parallel  ridges  between  the  valleys.  In  this 
way  there  may  be  formed  a  succession  of  ridges,  a  half 
dozen  or  more,  on  a  single  anticline.  Sometimes  one  hard 
layer  is  more  resistant  than  another  and  will  therefore 
form  a  higher  ridge,  the  less  resisting  layer  forming  a 
lower  parallel  ridge  which  to  the  observer  at  a  distance 
appears  like  a  terrace  on  the  side  of  the  higher  mountain. 
These  are  called  terraced  mountains  and  occur  in  a  num- 
ber of  places  among  the  Alleghany  ridges.     Tussey  Moun- 


PHYSIOGRAPHIC   FEATURES 


339 


tain  along  the  south  side  of  the  great  Nittany  Valley  in 
central  Pennsylvania  is  a  terraced  mountain.  (See  fig. 
241.) 


Fig.  241.  Tussey  Mt.,  Center  Co.,  Pa.  View  of  a  terraced  mountain. 
The  first  ridge  or  terrace  is  several  hundred  feet  lower  than  the  one  in 
the  background  and  is  separated  from  it  by  a  valley  eroded  on  the  softer 
sandstone  between  the  ridges.  The  stream  from  the  dividing  valley  flows 
through  the  water  gap  in  the  middle  of  the  picture. 


Canoe  Mountains.—  Canoe  mountains  are  formed  in  anticlinal 
and  synclinal  folds  by  the  ends  of  the  axis  dipping  below  the 
surface,  as  shown  on  the  accompanying  diagrams.  They  receive 
their  name  from  their  resemblance  to  a  large  upturned  canoe  in 
the  anticline  and  an  upright  canoe  in  the  syncline.  After  the 
erosion  of  the  top  of  an  anticlinal  fold  it  resembles  a  canoe  with 
the  bottom  worn  off.  Deep  erosion  in  the  central  portion  of  such 
a  fold  produces  some  unique  basins  or  coves  cut  off  or  isolated 
from  surrounding  regions  by  a  mountain-rim.  People  dwelling 
in  such  isolated  localities  are  likely  to  retain  habits  and  customs 
which  have  disappeared  years  before  in  more  cosmopolitan 
localities.     (Fig.  242.) 

A  great  mountain  range,  such  as  the  Alleghany,  consists  of 
a  series  of  simple  and  complex  folds  with  often  a  bewildering 


340 


PHYSICAL  GEOGRAPHY 


number  of  ridges,  all  of  which  go  to  make  up  the  mountain  range. 

264.  Domed  Mountains.—  Domed  mountains  are 
formed  by  the  uplift  of  a  broad,  dome-shaped  arch  instead 
of  an  elongated  one;  that  is,  the  layers  dip  from  a  point 


Fig.  242.  Canoe  Mountain  which  occur.r  among  the  Alleghany 
mountain  ridges.  H,  H,  hard  rock  strata  wnlch  form  ridges 
when  intervening  softer  layers  are  removed  by  erosion.  The 
upper  figure  is  a  syncline  in  which  the  hard  layers  resemble 
a  nest  of  canoes.  The  lower  figure  is  an  anticline  in 
which  the  canoes  are  inverted  and  the  bottom  worn  off. 
(After  Willis.) 


PHYSIOGRAPHIC   FEATURES  341 

or  center,  instead  of  from  a  line  as  in  the  anticlinal  folds. 
Domed  mountains  may  be  simple  or  complex  with  the  cen- 
tral mass  more  or  less  intricately  folded  or  faulted.  The 
Adirondack  Mountains  are  an  example  of  complexly 
crumpled  domed  mountains.  They  consist  of  crystalline 
metamorphic  and  igneous  rocks  surrounded  by  sedimen- 
tary layers  which  dip  away  from  the  mountains  in  all  di- 
rections. 

265.  Laccolites. — A  laccoUte  or  laccolithic  mountain  is  formed 
by  the  intrusion  of  a  mass  of  igneous  rock  which  does  not  reach 
the  surface  but  spreads  out  between  the  layers  in  a  mushroom- 
shaped  or  umbrella-shaped  mass,  at  the  same  time  pushing  or 
bulging  up  the  overlying  layers  into  a  dome-shaped  mass. 

Like  all  other  mountains,  the  laccolites  generally  have  a  very 
irregular  and  diversified  surface  due  to  the  action  of  eroding 
agents.  The  Henry  Mountains  in  Utah  are  good  examples  of 
laccolites  or  simple  domed  mountains.     (See  fig.  211.) 

266.  Block  Mountains  are  caused  by  fracturing  of  the  earth's 
crust  into  huge  blocks  which  are  then  tilted  or  set  on  edge, 
thus  giving  the  newly  formed  mountain  a  steep  slope  on  one  side 
and  a  quite  gentle  slope  on  the  other.  They  are  the  fragments 
of  a  broken-up  plateau.  They  occur  in  the  great  interior  basin 
east  of  the  Sierra  Nevada  Mountains. 

267.  Mountains  of  Circum-Erosion.—  Mountains  may  be 
formed  from  a  plateau  without  fracturing  by  the  eroding  action 
of  streams  and  their  tributaries,  which  cut  the  plateau  into 
fragments  that  are  called  mountains.  The  larger  ones  are  likely 
to  be  flat-topped  and  form  table  mountains.  They  are  very  irre- 
gular in  shape  and  size.  Such  are  the  Catskill  Mountains,  and 
the  mountains  of  western  Pennsylvania  and  West  Virginia.  (See 
Fig.  299.) 


Volcanic  Mountains.— Volcanic  mountains  con- 
sist of  the  cinder  and  lava  cones  built  up  around  the  crater 
of  a  volcano.  Some  of  the  loftiest  mountain  peaks  in  the 
world  are  formed  in  this  way.  While  the  volcano  is  still 
active  the  cone  may  be  quite  symmetrical,  but  as  soon  as 
it  becomes  extinct  the  effects  of  the  eroding  agencies  are 


342  PHYSICAL  GEOGRAPHY 

shown  in  the  gullies  and  valleys  that  form  on  the  slope. 
When  the  other  portion  of  the  mountain  is  worn  away  the 
central  neck  or  core  of  the  volcano  may  still  form  a  prom- 
inent mountain  mass.  (See  figs.  207,  208,  and  209.)  Mt. 
Shasta,  Mt.  Hood,  Mt.  Baker  and  many  other  high  moun- 
tain peaks  of  the  western  United  States  are  volcanic 
mountains. 

269.  Life  History  of  Mountains. — All  the  great  mountain 
ranges  are  born  in  the  sea,  having  a  beginning  in  the  accumula- 
tion of  a  great  mass  of  sediments  on  the  ocean  bottom  bordering 
the  continents.  Since  the  margin  of  the  ocean  is  shallow,  in 
order  to  have  a  great  thickness  of  deposits,  it  follows  that  the 
bottom  is  sinking  while  the  sediments  accumulate.  After  a  long 
period  of  subsidence  the  uplift  or  movement  in  the  opposite  di- 
rection begins,  when  the  sea  bottom  sediments  are  folded  and 
elevated  as  mountains  high  above  the  sea.  Then  the  agencies 
of  erosion,  the  sculpturing  agencies,  begin  their  work.  The  accu- 
mulation of  the  thick  bed  of  sediments  might  be  called  the 
embryonic  stage  of  development. 

The  youthful  stage  is  that  immediately  following  the  eleva- 
tion above  the  sea  in  which  the  mountain  has  the  regular 
slopes  of  the  first  uplift.  The  rain  and  the  other  weathering 
agencies  soon  start  gulches  and  valleys  which  change  the  smooth 
slopes  into  very  rugged  ones.  The  more  rapid  erosion  of  the 
softer  strata  causes  the  harder  layers  to  stand  out  as  hills,  ridges 
and  irregularities  on  the  surface.  During  this  stage  waterfalls, 
rapids,  canyons,  gorges,  steep  cliffs,  and  talus  slopes  are  formed. 
In  the  higher  mountains,  snow  fields  with  accompanying  glaciers 
and  glacial  erosion  modify  the  surface,  while  avalanches,  land- 
slides and  earthquakes  are  accompanying  features.  (See  figs. 
104,  114,  and  115.) 

Mature  stage.  The  mountains  pass  from  youth  to  maturity 
as  the  softer  parts  are  worn  away,  as  the  streams  are  established, 
as  the  ridges  and  peaks  reach  great  elevation  and  ruggedness, 
and  as  the  divides  become  narrow  and  well  defined.  The  valleys 
are  still  deep  but  wider  than  in  the  youthful  stage,  while  fiood 
plains  are  forming  on  the  short  grade-level  stretches,  talus  slopes 
are  becoming  larger  and  extending  nearer  the  tops  of  the  cliffs, 
waterfalls   are   fewer,   and   caves   are   forming  in   the   limestone 


PHYSIOGRAPHIC   FEATURES  343 

strata.  The  larger  streams  form  water  gaps  where  they  cut 
through  the  ridges  and  the  shifting  of  the  smaller  streams  leaves 
many  wind  gaps  which  serve  as  passes  for  highways. 

Old  age  begins  when  the  talus  slopes  extend  to  the  tops  of 
the  ridges  and  peaks,  which  are  crumbling  and  being  washed 
down  into  the  valleys,  when  the  flood  plains  are  expanding  and 
the  streams  meandering  in  their  courses.  The  heights  of  the 
mountains  are  lowered  by  erosion  from  the  summit  and  the  hill- 
sides are  made  less  steep  by  erosion  at  the  top  and  filling  in  at 
the  bottom.  The  talus  slopes  increase  in  size  until  they  reach 
and  cover  the  tops  o*f  the  cliffs.  The  rocky,  barren  cliffs  of 
maturity  give  way  to  soil-covered,  farm-covered  slopes  in  old  age. 

The  region  about  New  York  City,  Philadelphia  and  Balti- 
more represents  extreme  old  age  of  mountains,  where  they  have 
all  been  cut  down  to  low  hills  or  even  plains  in  places.  The  final 
stage  of  mountain  erosion,  like  that  of  plains  and  plateaus,  is  the 
peneplain  (Sec.  216)  which  may  in  time  be  brought  to  base  level. 
Over  the  peneplain  the  harder  and  more  resistant  rocks  stand 
forth  as  relict  mountains  or  monadnoclis,  the  remnants  of  the 
former  high  peaks  and  ridges.     (See  fig.  194.) 

Height.  The  height  of  a  mountain  or  mountain  range 
at  any  time  depends  upon  a  number  of  factors,  such  as  the 
rate  of  elevation,  the  character  of  the  rocks  in  the  moun- 
tain mass,  the  age  of  the  mountains,  and  the  climate. 

In  many  places  in  the  Alleghany  Mountains,  a  thick- 
ness of  several  miles  of  rock  has  been  eroded,  but  it  does 
not  follow  that  the  mountains  were  several  miles  higher 
than  at  present,  because  erosion  was  taking  place  while 
elevation  was  going  on.  Whether  the  mountains  were  ever 
higher  than  at  present  and  how  much  higher  depends  on 
the  relative  rate  of  activity  of  the  eroding  and  elevating 
forces. 

In  general  the  young  and  mature  mountains  are  the 
higher  mountains,  higher  because  they  are  young  and  ma- 
ture. The  age  of  mountains  is  commonly  reckoned  from 
the  time  of  their  first  uplift  from  the  sea  bottom,  which  is 
indicated  by  the  age  of  the  upper  strata  or  the  newest  sedi- 


344  PHYSICAL  GEOGRAPHY 

ments  that  take  part  in  the  folding  of  the  mountains.  It 
should  be  considered  that  some  of  the  mountain  ranges 
have  been  worn  down  and  re-elevated  one  or  more  times 
since  the  first  uplift.  There  is  good  reason  for  thinking 
that  the  Alleghany  Mountains  have  been  re-elevated  at 
least  twice. 

270.  Mountains  a?  Barriers.— Mountains  act  as  barriers  in 
the  distribution  of  moisture.  In  crossing  high  mountains, 
moisture-laden  winds  lose  most  of  the  water  on  the  windward 
side  and  pass  down  the  lee  side  as  drying  winds.  Thus  the  west 
slope  of  the  Andes  in  the  trade  wind  belt  receives  almost  no 
moisture,  as  it  is  nearly  all  precipitated  on  the  east  side  of  the 
mountains  and  helps  to  form  that  greatest  river  in  the  world,  the 
Amazon. 

The  higher  the  mountain,  the  more  effectual  barrier  is  it  to 
the  vegetation  and  the  lower  forms  of  animal  life.  Very  few  of 
the  plants  that  grow  on  the  plains  or  in  the  valleys  could  by 
natural  means  cross  such  mountains  as  the  Rocky  Mountains  or 
the  Andes.  Hence  the  native  plants — the  wild  flowers,  shrubs 
and  trees — are  quite  different  on  the  two  sides  of  these  moun- 
tains. The  same  is  true  of  many  animal  forms.  Some  of  the 
hardier  and  more  roving  forms  of  life  find  the  mountains  an 
obstruction,  but  like  man  they  can  and  do  cross  them. 

The  difficulties  attending  the  crossing  of  mountains  are 
frequently  a  decided  check  to  the  free  intercourse  of  the  peo- 
ple on  both  sides  and  the  partial  isolation  in  sequestered  moun- 
tain valleys  is  liable  to  cause  a  very  provincial  community,  in 
which  habits  and  customs  of  past  decades  are  preserved.  In 
many  of  the  deep  valleys  or  coves  in  the  Alleghany  Mountains 
one  may  see  the  customs  of  fifty  years  or  more  ago  with  little 
change  or  modification.  In  some  places  the  old  style  wagons  with 
wooden  axles,  linch  pins  and  tar  buckets  are  still  in  use.  These 
wagons  were  in  common  use  fifty  years  ago,  but  probably  none 
of  the  readers  of  this  book  ever  saw  one  unless  he  has  been  visit- 
ing in  some  of  the  sequestered  mountain  valleys. 

271.  Mountain  Climate.— The  climate  of  the  moun- 
tains is  different  from  that  of  the  surrounding  plains  and 
valleys.     So  marked  are  the  differences  on  mountains  of 


PHYSIOGRAPHIC  FEATURES  345 

even  moderate  height  from  surrounding  lowland  areas 
as  to  cause  a  difference  of  plant  and  animal  life.  The  cli- 
mate is  likely  to  prove  more  moist  than  on  the  lowlands, 
but  in  the  region  of  prevailing  winds  this  may  prove  true 
of  only  the  windward  side  of  the  mountains  while  the  lee 
side  may  be  dry  and  barren.  The  east  side  of  the  Andes 
in  the  trade  wind  belt  has  a  very  heavy  precipitation 
while  the  west  side  is  rainless.  In  the  Himalayas  the  north 
slopes  are  dry  and  are  bordered  by  an  arid  region,  while 
the  south  slopes  have  an  exceptionally  heavy  precipitation, 
the  heaviest  in  the  world. 

The  heavy  rainfall  and  snow  fall  of  the  mountains  are 
being  used  in  many  localities  in  the  western  United  States 
to  furnish  w^ater  for  irrigating  the  surrounding  semi-arid 
plains  and  plateaus.  The  mountains  and  plateaus  are  im- 
portant factors  in  inducing  rainfall  over  the  continents. 
The  winds  passing  over  the  ocean  and  plains  are  warmed 
and  absorb  moisture;  in  climbing  the  mountains  they  are 
cooled  and  precipitate  the  moisture. 

The  change  in  temperature  found  in  ascending  moun- 
tains is  quite  marked.  Not  only  is  the  average  tempera- 
ture lower,  but  the  daily  range  of  temperature  is  greater. 
Night  on  the  mountains  is  alw^ays  cool  and  on  the  highest 
mountains  is  alw^ays  cold  even  in  the  summer  season. 

272.  Economic  Features  of  Mountains.— The  moun- 
tains are  the  great  health  resorts,  furnishing  sites  for  sum- 
mer homes,  and  hotels,  where  fresh  air,  pure  water,  and 
wholesome  exercise  are  obtained  by  multitudes  from  the 
crowded  cities  and  towns. 

Timber  supply.  Some  of  the  mountains  are  utilized 
as  forest  preserves  and  more  of  them  should  come  under 
government  control  for  this  purpose.  The  most  rugged 
mountains  can  never  be  cultivated  to  advantage  and  where 
forests  are  not  preserved  or  cared  for,  the  mountains  be- 


346  PHYSICAL  GEOGRAPHY 

come  fire-swept,  barren  wastes  instead  of  profitable  and 
attractive  woodlands. 

The  forests  may  not  only  be  a  profitable  and  continued 
source  of  lumber  supply,  but  at  the  same  time  furnish 
game  preserves  where  wild  game  and  fish  may  flourish. 
They  may  at  the  same  time  furnish  the  much  needed  regu- 
lator of  water  supply  to  the  streams,  prevent  disastrous 
floods  in  one  5*eason  and  dry  stream  beds  in  another,  by 
conserving  the  heavy  rainfall  and  distributing  it  through 
the  dry  season. 

Mineral  wealth.  Mountains  are  the  sources  of  much 
of  the  mineral  wealth,  due  in  part  to  the  fact  that  deep 
erosion  has  exposed  the  deep  seated  rocks,  thus  causing 
the  exposure  of  a  great  thickness  and  range  of  rocks  to 
the  view  of  the  miner.  It  is  also  in  large  measure  due  to 
the  fracturing  and  metamorphism  of  the  rocks  during  the 
mountain-making  process,  thus  producing  more  veins  and 
greater  concentration  of  valuable  minerals  in  veins.  Most 
of  the  mines  of  gold,  silver,  quicksilver,  and  other  metals 
are  located  in  the  mountains,  occurring  in  old  mountains 
as  well  as  in  young  or  mature  ones. 

Building  stone.  There  are  large  exposures  of  rocks  of 
many  kinds  in  the  mountain  areas  because  of  the  folding 
and  erosion,  and  hence  they  furnish  sites  for  stone  quar- 
ries. However,  the  largest  and  most  productive  rock  quar- 
ries are  not  in  the  mountains.  The  limestone  quarries  of 
Indiana  and  Illinois,  the  sandstone  quarries  of  Ohio  and 
Connecticut,  the  granite  quarries  of  eastern  Massachusetts 
and  Southern  Maine  are  all  remote  from  mountains. 
What  reasons  can  you  give  for  such  a  condition  ? 

EEFERENCES 

Plains : 

Johnson,  High  Plains  of  the  United  States,   21st  An.  Rept. 
U.  S.  Geol.  Survey,  Pt.  VII,  p.  601. 


PHYSIOGRAPHIC  FEATURES  347 

Johnson,  High  Plains  of  the  United  States,  Natl.  Geog.  Mag., 

Vol.   IX,  p.   493. 
Salisbury,    The    Physical   Geography   of   New   Jersey,   N.   J. 

Geol.   Survey,  Trenton,  1895. 
Darton,  The  Great  Plains  of  the  Central  United  States,  An. 

Rept.  U.  S.  Geol.  Surv.,  also  Scottish  Geog.  Mag.,  Jan., 

1906. 
Deserts : 

Van  Dyke,  The  Desert,  Chas.  Scribner's  Sons,  1901. 
MacDougal,  Desert  Vegetation,  Publication  of  Carnegie  Insti- 
tute. 
National   Geographic   Magazine,   April,    1904,   The   American 

Deserts. 
Davis,  A  Temporary  Sahara,  Jour.  Sch.  Geog.,  Vol.  IV,  1900. 
Piatt,  The  Sahara,  Jour.  Sch.  Geog.,  Vol.  IV,  1900. 
Herbertson,  Man  and  His  Work. 
Plateaus : 

Powell,  Canyons  of  the  Colorado,  Flood  &  Vincent,  Meadville, 

Pa. 
Dutton,  The  Colorado  Canyon,  Mon.  II,  U.  S.  Geol.  Survey. 
Campbell    and    Mendenhall,   Plateau  of   West   Virginia,    17th 

An.  Rept.  U.  S.  Geol.  Surv. 
Hodge,   The   Enchanted   Mesa,  Natl.   Geog.   Mag.,  Vol.   VIII, 

1897,  p.  273. 
Mountains  : 

LeConte,  Theories  of  the  Origin  of  Mountain  Ranges,  Jour. 

Geol.,  Vol.  I,  p.  542. 
Willis,  Mechanics  of  Appalachian  Structure,  13th  An.  Rept., 

iPt.  II,  U.  S.  Geol.  Survey,  p.  217. 
Willis,  Northern  Appalachians,  Natl.  Geog.  Mon.,  Am.  Book 

Co.,  1895. 
Hayes,  Southern  Appalachians,  Natl.  Geog.  Mon.,  Am.  Book 

Co.,  1895. 
Davis,  Southern  New  England,  Natl.  Geog.  Mon.,  Am.  Book 

Co.,  1895. 
Davis,  Rivers  and  Valleys  of  Pennsylvania,  Nat.  Geog.  Mag., 

Vol.  I,  p.  183. 
Geikie,  Classification  of  Mountains,  Scot.  Geog.  Mag.,  v.  17, 

Sept.,  1901. 
Lubbock,  Scenery  of  Switzerland,  MacMillan  Company,  1896. 
Cross,  Laccolitic  Mountain  Groups,  14th  An.  Rept.  U.  S.  Geol. 

Surv.,  Pt.  II,  p.  165. 


CHAPTER  X 
THE  ATMOSPHERE 

273.  The  atmosphere  is  the  gaseous  portion  of  the 
earth  which  surrounds  the  liquid  and  solid  portions. 
Floating  in  suspension  in  the  atmosphere  are  large  but 
variable  quantities  of  moisture  in  the  form  of  invisible 
vapor  and  also  condensed  vapor  in  the  form  of  clouds,  fog 
or  mist,  along  with  many  dust  particles  and  microscopic 
organisms.  All  this  material  while  in  the  atmosphere  may 
be  considered  as  a  part  of  it,  in  the  same  way  that  the  por- 
tions of  the  gases  which  penetrate  the  water  and  the  land 
may  be  considered  as  portions  of  the  water  and  land- 
spheres  for  the  time.  The  air  is  as  truly  a  part  of  the 
earth  as  the  water  or  the  land.  It  even  approaches,  pos- 
sibly exceeds  them  in  volume,  although  it  is  less  in  mass. 

The  science  which  treats  of  the  atmosphere,  its  posi- 
tion, functions,  phenomena  and  laws  governing  them  is 
called  meteorology. 

274.  Origin  of  the  Atmosphere.— According  to  the  nebular 
hypothesis,  all  portions  of  the  earth  were  at  one  time  in  a  gas- 
eous condition,  but  as  the  gases  cooled  and  contracted,  the 
greater  part  became  liquid,  most  of  which  on  further  cooling  be- 
came solid.  The  air  is  the  portion  which  still  remains  gaseous. 
The  physical  state  of  matter  is  partly  a  question  of  temperature. 
At  32  degrees  F.  (F.=Fahrenheit,  the  thermometer  in  com- 
mon use)  water  freezes  and  becomes  solid;  at  212  degrees  it 
boils  and  becomes  a  gas.  Some  substances,  as  metallic  mercury, 
freeze  at  a  much  lower  temperature,— 40  degrees  F.,  while  many 
of  the  common  rocks  freeze  solid  at  temperatures  as  high  as 
3,000  degrees  to  5,000  degrees  F.  Portions  of  the  present  at- 
mosphere have  been  reduced  to  a  liquid  by  lowering  the  tempera- 

348 


THE    ATMOSPHERE  349 

ture  and  increasing  the  pressure,  but  at  ordinary  temperatures 
it  remains  gaseous. 

275.  Function  of  the  Air.— In  the  economy  of  the 
earth  the  atmosphere  serves  many  important  functions, 
such  as  (1)  diffusing  light;  (2)  conducting  sound;  (3) 
retaining  heat;  (4)  supporting  life  in  many  ways;  (5) 
supporting  combustion;  (6)  moving  ships;  (7)  driving 
windmills;  (8)  reducing  the  weight  of  bodies  submerged 
in  it,  thus  making  it  possible  for  some  animals  to  walk  and 
others  to  fly;  (9)  producing  waves  and  ocean  currents; 
(10)  moving  sand  and  dust  and  wearing  away  rock;  (11) 
distributing  moisture  and  heat.  What  other  functions 
can  you  name?  Can  you  see  the  air?  Can  you  feel  it? 
Can  you  weigh  it? 

276.  Composition  of  the  Atmosphere.— The  atmos- 
phere consists  of  a  mixture  of  nitrogen  and  oxygen  in  the 
proportion  of  nearly  four  parts  of  nitrogen  to  one  of  oxy- 
gen along  with  small  but  variable  quantities  of  carbonic 
acid  gas,  water  vapor,  argon,  crypton,  helium,  and  prob- 
ably other  rare  but  yet  unknown  gases,  besides  variable 
quantities  of  dust  particles. 

Nitrogen,  which  forms  about  four  fifths  of  the  bulk  of  the 
atmosphere,  is  one  of  the  most  inert  of  the  gases  and  so  far 
as  life  is  concerned  its  chief  function  appears  to  be  to  dilute 
the  oxygen.  It  has  little  or  no  tendency  to  combine  with  the 
other  elements  under  normal  conditions,  but  certain  plants,  such 
as  clover,  have  the  power  of  secreting  it  in  their  roots  in  the 
form  of  nitrates  which  greatly  enrich  the  soil.  Nitrogen  com- 
bined chemically  with  oxygen  and  water  forms  nitric  acid.  If 
this  comes  in  contact  with  soda  or  potash  it  combines  with  them 
forming  compounds  called  nitrates. 

Oxygen  forms  about  one  fifth  of  the  atmosphere  and  is  much 
more  active  and  aggressive  in  its  character  than  the  nitrogen. 
It  is  the  chief  agent  in  combustion;  in  fact  nearly  all  burning 
consists  of  the  chemical  union  of  oxygen  with  carbon,  forming 
carbon  dioxide  and  other  gases,  whether  it  be  in  fires,  where  it 


350  PHYSICAL  GEOGRAPHY 

produces  heat  and  light,  or  in  our  own  bodies  where  it  forms 
heat,  but  not  light.  Oxygen  is  ever  active  also  in  combining 
with  metals  and  minerals  in  the  rocks  of  the  earth.  A  knife  left 
on  the  ground  in  a  few  days  is  covered  with  rust;  in  a  few 
months  it  crumbles  to  fragments,  eaten  up  by  the  rust,  in  other 
words,  by  the  oxygen  of  the  atmosphere.  Oxygen  forms  half  of 
the  rocks  of  the  earth's  crust,  and  eight-ninths  of  the  water.  It 
is  being  taken  from  the  atmosphere  by  animals,  by  fires,  by  the 
rusting  of  rocks  and  minerals.  It  is  being  returned  by  plants 
which  take  the  carbon  dioxide  gas  from  the  air  and  separate  it 
into  the  elements,  carbon,  which  makes  new  compounds  forming 
part  of  the  plant,  and  oxygen,  which  is  returned  to  the  air.  It 
is  also  set  free  by  certain  chemical  changes  in  the  rocks. 

Carbon  dioxide  or  carbonic  acid  gas  (CO2)  forms  a  small  but 
important  part  of  the  air.  The  proportion  varies  greatly  at 
different  places  and  times  but  averages  about  0.03%. 

It  is  one  of  the  chief  heating  agents  in  the  atmosphere  and 
thus  has  considerable  influence  on  the  climate,  owing  to  a  varia- 
tion in  the  proportion  present  from  time  to  time.  It  is  a 
denser,  heavier  gas  than  nitrogen  or  oxygen  and  absorbs  and 
holds  the  heat  from  the  sun's  rays,  thus  serving  as  a  blanket 
to  warm  the  earth.  It  is  directly  necessary  to  plant  life,  furnish- 
ing the  most  important  article  of  food  for  the  plant  and  indirectly 
necessary  to  animal  life.  Why?  When  dissolved  in  water  it 
becomes  an  active  agent  of  solution  and  disintegration  in  the 
rocks,  especially  the  limestones. 

Carbonic  acid  is  added  to  the  atmosphere  by  the  breath  of 
animals,  by  the  decay  and  combustion  of  all  animal  and  vegetable 
matter,  by  volcanoes,  by  carbonated  springs  and  from  the  sea 
water.  It  is  extracted  from  the  air  by  plants  which  fix  the  car- 
bon in  their  tissue  and  giv«  the  oxygen  back  to  the  air,  by  the 
disintegration  of  the  rocks  in  which  it  combines  with  other  ma- 
terials to  form  carbonates,  and  it  is  absorbed  by  the  sea  water 
and  fresh  waters  where  these  are  not  already  saturated  with  it. 
Owing  to  this  circulation  through  the  rocks  and  the  sea,  the 
proportion  of  this  gas  in  the  air  varies  sufficiently  from  one  geo- 
logic age  to  another,  it  is  thought,  to  affect  the  climate  materially. 

Water  vapor  is  a  small,  variable,  but  very  important  consti- 
tuent of  the  air.  The  water  in  an  invisible  gaseous  condition  is 
absorbed  by  the  atmosphere  from  the  surface  of  the  ocean,  other 


THE    ATMOSPHERE  351 

bodies  of  water,  and  the  moist  land.  The  gases  ejected  from 
volcanoes  also  supply  large  quantities  of  vapor  to  the  air.  An- 
other source  of  supply  is  in  combustion  and  as  exhalation  from 
the  lungs  of  animals. 

The  water  in  the  invisible  gaseous  state  is  carried  by  the 
winds  over  the  continents,  where,  condensing  in  clouds,  it  is  pre- 
cipitated in  the  form  of  rain  or  snow  to  fall  on  the  earth  and 
flow  back  to  the  ocean  again  to  start  on  a  similar  circuit.  The 
circulation  of  water  in  this  way  is  essential  to  all  life — in  the 
ocean,  in  the  air,  and  on  the  land.  A  large  per  cent  of  all  living 
plant  and  animal  matter  consists  of  water  and  there  must  be  a 
constant  renewal  of  the  water  supply  in  order  to  retain  the  life. 

Dust  is  another  important  constituent  of  the  air  which  is 
everywhere  present  but  most  abundant  over  the  land  areas  near 
the  surface  in  dry  weather.  Why?  It  consists  of  exceedingly 
fine  particles  of  pulverized  rock  carried  up  by  the  winds  or 
blown  out  from  volcanoes,  of  particles  of  unconsumed  fuel  in  the 
smoke,  of  living  germs  in  the  form  of  bacteria  and  microbes,  or 
of  decayed  plant  and  animal  tissue.  In  a  ray  of  sunshine  passing 
through  a  small  opening  into  a  dark  room,  one  can  see  a  vast 
number  of  the  dust  particles,  but  there  is  a  much  larger  number 
invisible  because  so  very  small. 

The  dust  of  the  air  is  thought  to  be  an  important  aid  in  the 
precipitation  of  moisture.  The  dust  particle  becomes  a  center 
of  condensation  of  moisture  until  there  is  sufficient  to  form  the 
raindrop  or  the  snowflake,  which  then  falls  to  the  earth.  It  acts 
with  the  carbonic  acid  and  the  moisture  in  heating  the  air,  as 
each  little  dust  particle  becomes  a  little  furnace  or  reservoir  of 
heat  which  it  radiates  in  all  directions.  The  bacterial  portior 
aids  decomposition  and  the  spread  of  disease.  It  influences  the 
color  of  the  sky,  the  brilliant  red  sunset  being  due  largely  to  the 
dust  particles  in  the  air. 

Dust  is  sometimes  classed  as  an  impurity  in  the  atmosphere, 
but  since  it  is  always  present,  and  serves  a  useful  if  not  a  neces- 
sary purpose  for  the  support  of  life,  it  is  properly  one  of  the 
constituents  of  the  air. 

277.  Pressure  of  the  Air.- Because  the  pressure  of 
the  air  is  exerted  equally  in  all  directions,  it  long  remained 
unperceived.  If  we  should  exhaust  all  the  air  from  under- 
neath a  scale  pan,  we  would  find  a  weight  of  air  on  the 


352 


PHYSICAL  GEOGRAPHY 


pan  of  about  fifteen  pounds  to  the  square  inch  or  a  little 
more  than  a  ton  to  the  square  foot,  that  is,  provided  the 
weight  were  taken  at  sea  level ;  with  change  of  temperature 
or  change  of  elevation  the  pressure  would  change.  It 
would  increase  if  taken  below  sea  level  and  decrease  if 
taken  above.  It  would  decrease  with  an  increase  in  the 
temperature  and  increase  with  a  decrease  in  temperature 
because  the  warm  air  expands  and  is  therefore 
lighter  than  the  same  volume  of  cold  air  subject 
to  the  same  pressure  and  the  cold  air  contracts 
and  is  hence  heavier  than  a  similar  volume  of 
warm  air. 

Weight  on  the  earth  is  the  effect  of  gravity 
which  tends  to  draw  all  bodies  toward  the  center  of 
the  earth,  hence  the  weight  of  the  atmosphere  at 
any  point  is  its  vertical  pressure,  but  owing  to  the 
extreme  mobility,  this  pressure  due  to  weight  is 
exerted  equally  in  all  directions,  so  that  under  ordi- 
nary conditions  we  do  not  find  the  scale  pan  pressed 
downward  by  air,  because  there  is  the  same  pressure 
underneath  as  on  top.  The  pressure  of  ten  tons  or 
more  on  the  outside  of  the  human  body  is  not  felt 
because  there  is  a  corresponding  pressure  on  the  in- 
side. The  weight  of  a  cubic  foot  of  air  at  sea  level 
at  a  temperature  of  60  degrees  is  .075  lbs.,  while  the 
pressure  exerted  by  this  cubic  foot  would  be  more 
than  a  ton  on  each  side. 

278.  Barometer.— The  instrument  com- 
monly used  in  weighing  the  air,  that  is,  de- 
termining the  pre.ssure,  is  a  barometer,  which  in 
its  j-implest  form  consists  of  a  glass  tube  closed 
at  one  end,  filled  with  mercury  and  inverted  in 
^, a  cup  of  the  same  metal.     If  this  is  done  at 

t  Hi.    '24'.i.  • 

Mercury       sea  Icvcl  the  mcrcury  will  settle  in  the  tube  until 
barometer,      ^hc  top  of  it  is  .ibout  30  iuchcs  abovc  the  level 

common 

form.         of  that  in  the  cup.    Now  a  column  of  mercury 


THE    ATMOSPHERE  353 

30  inches  high  weighs  15  pounds  per  square  inch  at  the 
base  of  the  cohimn,  hence  the  weight  of  the  air  pressing 
on  the  surface  of  the  cup  outside  of  the  tube  must  be  equal 
to  15  pounds  per  square  inch,  since  the  two  balance  each 
other.  Why  is  mercury  used?  Why  not  some  other 
liquid?  If  water  were  used  how  long  a  tube  would  it 
require  ? 

An  aneroid  barometer  differs  from  a  mercurial  one  in  substi- 
tuting for  the   column   of  mercury  a   small   corrugated    metallic 
box  from  which  the  air  has  been  exhausted  as 
nearly  as  possible.     The  surface  of  this  box  is 
connected  through  a  series  of  levers  to  a  needle 
in  such  a  way  that  when  the  sides  of  the  box 
are  pressed  in,  as  would  be  the  case  with  an 
increase  of  pressure  in  the  air,  the  needle  will 
turn  around  on  a  dial  like  the  hand  of  a  watch, 
and   when   the   pressure   decreases   the    needle 
moves  in  the  opposite  direction.    The  dial  of 
the  aneroid   is   marked   so  that  movements  of      j,        .^^^       ^^^ 
the  needle  will  indicate  the  number  of  feet  the  roid     barometer 

barometer  has  been  taken  up   or  down,  or  it  for     measuring 

may  be  marked  in  inches  to  correspond  to  the  elevations.     See 

movements  of  the   mercury  in  the  other  baro-  ^^^'  ^"^^  ^"^  ^"' 

„      ^     .^  „       ,  ,     ^,  ,  tenor    construc- 

meter;    m    fact    it    generally    has    both    scales.  ^j^^ 

Because  it  can  be  carried  in  the  pocket  like  a 

watch,  and  is  much  more  convenient  for  ordinary  use  than  the 

bulky  mercurial  barometer. 

279.  Density  of  the  Air.— If  a  barometer  were  car- 
ried up*  a  mountain  the  mercury  would  gradually  fall  in 
the  tube,  because  of  decrease  in  the  pressure  of  the  atmos- 
phere, until  at  an  elevation  of  3.4  miles  above  the  sea  there 
would  be  only  15  inches  in  the  tube ;  that  is,  at  that  height 
the  air  would  be  only  half  as  dense  or  heavy  as  at  sea 
level.  At  greater  elevations  the  density  has  been  esti- 
mated as  follows: 


23 


354 


PHYSICAL  GEOGRAPHY 


Elevation     6.8  miles,  density  ^  and  barometer  7.5     inches 

10.2  "             "  %                "  3.75 

13.6  "             "  T^                "  1.87 

17.0  "             "  ^V               "  -95 

This  estimate  is  based  on  the  assumption  that  the  den- 
sity decreases  in  the  upper  atmosphere  at  the  same  rate 
that  it  does  in  the  lower  atmosphere ;  that  is,  a  decrease  of 
one-half  for  every  3.4  miles  ascent. 

280.  Height  of  the  Atmosphere.-  The  upper  limit  of 
the  atmosphere  is  not  known,  but  various  estimates  have 
placed  it  at  heights  varying  from  50  to  "500  miles.  It  prob- 
ably extends  much  higher  than  either  figure ;  in  fact,  there 
is  good  reason  for  thinking  that  it  extends  as  far  as  the 
outer  limit  of  the  zone  of  control  of  the  earth's  gravity. 
But  if  the  decrease  in  density  approximates  that  indicated 
in  the  above  table  it  becomes  so  rare  as  to  be  difficult  of 
detection  by  any  available  means  far  short  of  200  miles. 

281.  Pressure  Curve.— If  the  barometer  readings  are 
recorded  at  any  point  for  several  days  it  will  be  seen  that 
there  is  considerable  variation.  If  the  barometer  should 
be  read  for  several  different  hours  each  day  for  several 


Fig.  245.  A  barogram  or  pressure  curve  made  by  a  barograph  during  the 
passage  of  a  cyclone  followed  by  a  cold  wave.  Notice  the  rapid  fall  and 
rise  of  pressure. 


days  and  the  results  plotted  on  cross-section  paper,  the  re- 
sult would  be  the  pressure  curve.  This  pressure  curve  is 
plotted  even  more  accurately  by  an  instrument  called  the 


THE    ATMOSPHERE 


355 


barograph.     (See  fig.  246.)     The  curve  made  by  the  baro- 
graph is  called  a  barogram.     (Fig.  245.) 

282.  Barograph.— The  barograph  used  by  the  U.  S,  Weather 
Bureau  is  based  on  the  principle  of  the  aneroid  instead  of  the 
mercurial  barometer.  It  consists  of  a  corrugated  iron  box  (B) 
from  which  the  air  has  been  exhausted  so  that  an  increase  in  the 


Fig.  246.  A  barograph.  The  pen  traces  a  line  on  the  paper  which  is  moved 
by  clockwork.  If  the  pressure  were  uniform  the  line  traced  would  be 
parallel  to  the  horizontal  lines  on  the  paper. 


pressure  of  the  air  depresses  the  surface  of  the  box  and  a  de- 
crease in  the  pressure  causes  a  corresponding  elevation  in  the 
surface  produced  by  a  spring  inside  the  box.  These  movements 
of  the  surface  are  magnified  by  a  series  of  compound  levers  to 
which  is  attached  a  pen  so  adjusted  as  to  leave  its  trace  on 
the  roll  of  cross-ruled  paper  which  is  moved  by  clockwork. 
This  shows  conclusively  that  the  pressure  at  any  place  at  any 
one  time  is  dependent  on  the  condition  of  the  weather.  If  the 
effect  of  the  weather  on  the  barometer  is  known,  then  the  process 
may  be  reversed  and  the  record  of  the  barometer  may  be  taken 
as  the  indication  of  the  weather  conditions;    and  if  ajong  with 


356  PHYSICAL    GEOGRAPHY 

this,  the  regular  movements  of  the  atmosphere  are  considered, 
the  weather  conditions  may  often  be  foretold  for  some  time 
ahead. 

283.  Isobars.— In  order  to  compare  barometer  read- 
ings from  different  localities  to  determine  probable  weather 
conditions,  it  is  necessary  to  make  corrections  for  the  dif- 
ferences in  elevation  of  the  places  and  another  for  the  dif- 
ferences in  temperature.  Tables  have  been  made  out  for 
this  purpose  so  that  having  the  reading  of  the  barometer 
and  the  thennometer,  and  knowing  the  elevation  of  the 
point  above  sea  level,  it  is  only  necessary  to  turn  to  the 
tables  and  make  the  necessary  additions  or  subtractions  to 
reduce  the  readings  from  different  places  to  a  common 
plane,  which  by  common  consent  is  taken  as  sea  level.  The 
records  that  are  given  on  the  government  weather  maps 
are  all  the  corrected  sea-level  readings  at  the  temperature 
of  32  degrees  F. 

In  order  to  bring  out  graphically  the  results  obtained 
from  barometric  readings  at  many  different  stations,  lines 
representing  variations  in  pressure  of  one-tenth  of  an  inch 
are  drawn  through  points  having  the  same  barometric 
pressure.  Such  lines  are  called  isobars  (meaning  equal 
pressure),  and  are  shown  on  the  daily  weather  maps  by 
continuous  black  lines.  Compare  carefully  several  daily 
isobaric  charts  or  weather  maps  with  each  other  and  with 
the  isobaric  chart  of  the  world  for  the  year.     (See  fig.  254.) 

284.  Barometric  Gradients.— On  the  daily  weather  map  most 
of  the  isobars  are  more  or  less  concentric  around  centers,  some 
of  which  are  marked  high  and  some  marked  low,  meaning  high 
pressure  and  low  pressure.  The  barometric  gradient  is  the  rate 
at  which  the  air  pressure  changes  from  place  to  place  and 
particularly  between  the  high  and  the  low.  It  indicates  the 
direction  in  which  the  atmosphere  tends  to  move,  namely, 
from  the  high  to  the  low,  and  the  steeper  the  grade,  that  is, 
the  greater  the  number  of  the   isobars,   the   more   rapidly  the 


THE    ATMOSPHERE  357 

air  moves   and   hence  the   stronger   is   the  wind.     Test  this  by- 
comparing  current  weather  maps  on  a  windy  day  and  a  calm  one. 

285.  Temperature  and  Heat.— Temperature  is  the 
measure  of  the  heat  energy  of  any  body.  The  instrument 
used  for  recording  the  temperature  is  called  a  thermometer 
(heat  measure).  In  the  common  house-thermometer  mer- 
cury rises  and  falls  through  a  capillary  tube  from  expan- 
sion and  contraction  with  increasing  and  decreasing  tern-* 
perature,  and  generally  indicates  increase  and  decrease  of 
heat.  However,  many  bodies  are  capable  of  absorbing  or 
giving  off  considerable  heat  without  any  perceptible  change 
of  temperature.  Thus  when  heat  is  applied  to  ice,  the 
temperature  does  not  vary  until  all  the  ice  has  been  changed 
to  water.  The  heat  required  to  change  a  pound  of  ice  to 
water  at  the  same  temperature  would  raise  the  pound  of 
water  from  32  degrees  to  174  degrees  F.  This  is  called 
latent  heat  of  fusion,  and  will  be  given  off  again  before 
the  water  will  freeze.  It  requires  a  large  increment  of 
heat  to  change  water  from  a  liquid  at  212  degrees  to  a 
vapor  at  the  same  temperature,  the  latent  heat  of  vapori- 
zation. (This  is  technically  expressed  in  heat  units  called 
calories.  A  calorie  is  the  amount  of  heat  required  to  raise 
a  gram  of  water  one  degree  Centigrade.  The  latent  heat 
of  vaporization  of  water  is  536.6  calories.)  Why  does 
sprinkling  the  lawn  or  the  street  on  a  hot  summer  day  cool 
the  air?  Why  is  it  generally  warmer  in  winter  and  cooler 
in  summer  on  the  sea  shore  than  in  the  interior?  Why  is 
it  sometimes  warmer  after  a  rain  than  before? 

286.  Thermometer.- The  temperature  of  the  air  is 
usually  measured  by  a  mercury  thermometer,  which  con- 
sists of  a  small  bulb  filled  with  metallic  mercury  that  is 
attached  to  a  capillary  tube  in  which  the  mercury  rises 
when  it  is  heated  and  in  which  it  sinks  when  it  is  cooled. 
On  the  tube  or  on  a  fiat  surface  to  which  the  tube  is  at- 


358 


PHYSICAL    GEOGRAPHY 


taehed  is  a  scale  marked  off  in  degrees  from  which  is  read 
the  number  corresponding  to  the  height  of  the  mercury  in 
the  tube.  The  mercury  thermometer  may  be  used  for 
measuring  temperatures  between  — 40  degrees,  the  freezing 
point  of  mercury,  and  648  degrees  F.,  the  boiling  point. 
For  lower  temperatures  some  other  liquid  as  alcohol  or 
ether  is  used ;  for  higher  temperatures  some  more  resistant 
metal  or  metals  or  some  other  device  is  used.  Describe  a 
maximum  and  a  minimum  thermometer  and  the  Fahren- 
heit, Centigrade,  and  absolute  scales  for  grading  ther- 
mometers, if  these  instruments  are  at  hand. 

287.  Temperature  Curve. —The  variation  in  temper- 
ature at  any  place  from  time  to  time  may  be  represented 
by  a  temperature  curve  as  shown  in  fig.  247.     The  curve 


Fig.   247.     Thermogram,   a  temperature   curved   formed  by   the   thermograph. 


may  be  constructed  for  any  period  of  time  for  which  the 
thermometer  readings  have  been  taken.  The  thermograph 
automatically  records  such  a  curve  with  greater  accuracy 
of  detail  than  can  be  shown  on  one  drawn  from  thermo- 
meter readings.  Study  curves  of  this  kind  if  available, 
and  note  the  time  of  day  when  the  maximum  and  mini- 
mum temperatures  occur.  Each  student  should  plot  a 
temperature  curve  for  a  week  or  a  month  from  data  ob- 
tained by  reading  the  thermometer  and  recording  the  tem- 
perature 3  or  more  times  each  day. 


THE    ATMOSPHERE 


359 


A  thermograph  is  an  instrument  for  recording  automatically 
the  temperature  for  a  fixed  period  of  time  in  much  the  same  way 
as  a  barograph  records  pressure.  The  record  of  the  thermograph 
is  a  thermogram  or  temperature  curve.     (See  fig.  247.) 


Fig.    248.       Thermograph,    an    instrument    for    recording    temperature    changes. 


288.  Sources  of  Heat.— The  three  sources  of  heat  sup- 
ply on  the  earth  are  (1)  the  sun,  (2)  the  stars  and  other 
heavenly  bodies,  (3)  the  internal  heat  of  the  earth.  In 
quantity  the  last  two  are  insignificant  when  compared  with 
that  from  the  sun,  which  is  the  source  of  not  only  nearly 
all  the  heat,  but  the  light,  and  other  forms  of  energy  as 
well.  The  radiant  energy  that  comes  from  the  sun  is 
known  as  insolation,  which  manifests  itself  on  the  earth  in 
part  as  heat,  in  part  as  light,  and  probably  in  several  other 
forms  of  energy.  The  earth  receives  about  one  two-bil- 
lionth portion  of  the  sun's  insolation  and  a  large  part  of 
that  received  is  not  retained  but  is  reflected  or  radiated 
off  into  space. 


360  PHYSICAL    GEOGRAPHY 

A  small  portion  of  the  sun's  rays,  as  they  pass  through  the 
atmosphere,  are  intercepted  by  the  dust  particles  and  the  heavier 
gases  and  changed  to  sensible  heat,  but  the  greater  part  passes 
directly  through  to  the.  surface  of  the  earth,  where  a  portion  is 
absorbed  by  the  rock-and-water-surface  and  changed  to  latent  or 
radiant  heat  and  a  portion  is  reflected  back  through  the  atmos- 
phere. The  proportion  of  rays  absorbed  to  those  reflected,  varies 
greatly  with  different  surfaces.  More  are  reflected  from  a  water 
surface  than  from  rock  and  more  from  light-colored  rocks  than 
from  dark-colored  ones.  There  is  likewise  a  wide  variation  de- 
pending on  the  angle  of  inclination  of  the  rays  to  the  surface. 
The  greatest  percentage  of  insolation  is  absorbed  under  the  ver- 
tical rays  and  as  they  vary  from  the  vertical  there  is  an  increas- 
ing proportion  reflected  until  the  tangent  rays,  such  as  those  at 
sunrise  and  sunset,  are  nearly  all  either  reflected  or  else  pass 
directly  through  the  atmosphere  into  space  beyond. 

289.  Temperature  of  the  Air.— The  atmosphere  is 
warmed  (1)  by  the  direct  insolation  of  the  sun,  (2)  from 
the  heated  surface  of  the  earth  by  radiation,  by  conduc- 
tion, and  by  convection,  and  (3)  by  compression  as  shown 
when  pumping  air  into  a  bicycle  tire.  When  the  air 
descends  from  altitudes  of  less  pressure  to  regions  of  greater 
pressure  it  is  compressed  and  warmed,  as  in  the  high  pres- 
sure areas  and  where  the  air  flows  from  the  mountain  top 
into  the  valley  or  plain ;  (4)  by  precipitation.  The  great 
quantity  of  heat  required  to  change  water  to  vapor  remains 
latent  in  the  vapor  and  is  given  oft*  when  the  moisture  is 
precipitated. 

The  air  is  cooled  (1)  by  radiation  of  its  heat  into  space; 
(2)  by  conduction  when  it  comes  into  contact  with  the 
cooler  earth  or  body  of  water;  (3)  by  expansion  as  when 
it  flows  out  of  a  bicycle  tire  or  when  it  rises  from  a  region 
of  greater  to  one  of  less  pressure  as  in  ascending  the  moun- 
tain or  in  the  rising  currents  in  the  low  pressure  areas; 
(4)  by  convection,  as  by  the  descent  of  the  cold  currents 
to  replace  the  rising  currents  of  heated  air;  (5)  by  evapor- 


THE    ATMOSPHERE  361 

ation.  How?  The  heating  of  the  air  by  compression  and 
the  cooling  by  expansion  is  known  as  adiahatic  heating  and 
cooling. 

290.    Elements  Affecting  the  Temperature  of  the  Air. 

— The  distribution  of  solar  heat  is  determined  by  the  suc- 
cession of  day  and  night  and  of  the  seasons.  The  atmos- 
phere and  the  earth  underneath  it  receive  heat  directly 
from  the  sun  during  the  day,  receiving  the  greatest  quan- 
tity at  midday  when  the  rays  are  most  nearly  vertical. 
The  temperature,  however,  is  the  highest  from  one  to  two 
hours  after  noon.  Why?  There  is  more  or  less  regular 
decrease  in  the  heat  received  from  the  sun,  as  one  goes 
from  the  tropics  toward  the  poles.     Why? 

Efect  of  pressure.  In  areas  of  high  pressure,  the  air  is  being 
warmed  because  it  is  under  pressure  and  in  areas  of  low  pres- 
sure, it  is  being  cooled  because  it  is  rising  and  expanding.  How- 
ever, the  temperature  of  the  air  near  the  earth  is  higher  in  the 
low  pressure  areas  than  in  the  high;  in  fact,  that  is  the  reason 
why  it  is  low,  because  the  temperature  is  higher  and  hence  the 
air  is  lighter  and  is  crowded  up  by  the  inward  pressure  of  the 
surrounding  heavier  air;  the  temperature  is  kept  high  because 
the  air  moves  into  the  center  along  the  surface  of  the  earth, 
where  it  is  warmed  by  conduction  and  frequently  by  precipita- 
tion of  the  moisture;  it  cools  by  expansion  as  it  rises,  but  it  is 
then  beyond  the  reach  of  our  senses.  The  air  is  cooler  and  feels 
cooler  in  the  high  centers  because  it  is  descending  from  the 
higher  altitudes,  where  it  is  much  colder.  The  clear  air  of  the 
high  center  favors  radiation  of  heat  and  in  that  way  lowers  the 
temperature.  Cold  waves  come  with  the  high  centers  in  the 
winter.  The  fact  that  the  air  is  warmed  in  the  high  and  cooled 
in  the  low  is  shown  in  two  ways,  by  comparing  the  temperature 
taken  at  high  altitudes,  by  means  of  a  kite  or  balloon,  and  second 
by  noting  that  in  the  highs  the  air  is  clear  and  generally  free 
from  clouds  and  rain,  while  in  the  lows,  it  is  cloudy  and  fre- 
quently precipitates  rain  or  snow.  Cooling  the  air  condenses  the 
moisture  and  warming  it  increases  its  capacity  for  moisture, 
causing  it  to  dissolve  the  clouds  instead  of  condensing  them. 
(See  Sec.  296  and  314  for  explanation  of  high  and  low  pressures.) 


362  PHYSICAL    GEOGRAPHY 

Efect  of  latitude.  The  seasonal  changes  are  most  marked  at 
and  near  the  poles  and  least  at  and  near  the  equator,  due  to  the 
fact  of  less  variation  in  the  inclination  of  the  sun's  rays  in  the 
tropics  and  greater  uniformity  in  the  distribution  of  sunlight 
and  darkness. 

The  temperature  decreases  with  an  increase  of  latitude  at 
the  average  rate  of  about  1  degree  F.  for  1  degree  or  about  70 
miles  of  latitude. 

Efect  of  altitude.  The  temperature  decreases  with  altitude 
at  the  average  rate  of  about  1  degree  F.  for  every  300  feet  of 
ascent;  hence,  at  the  equator  one  would  find  about  the  same 
change  in  temperature  by  ascending  a  mountain  six  miles  high 
as  in  going  north  or  south  1000  times  this  distance.  The  reason 
for  the  rapid  decrease  in  temperature  in  ascending  the  moun- 
tain is  that  the  air  is  less  dense,  and  contains  less  carbon  dioxide, 
water  vapor,  and  dust  particles  that  serve  to  warm  the  air  at 
lower  altitudes. 

Effect  of  'Wa.ter.— Bodies  of  Water  tend  to  equalize 
the  temperature  of  the  region  bordering  them,  because  they 
absorb  heat  less  rapidly  than  the  land,  do  not  become  so 
hot  during  the  day  or  during  the  summer  season,  and 
since  they  part  with  their  heat  less  rapidly  than  the  land, 
they  do  not  cool  as  quickly;  hence  the  water  is  cooler  than 
the  land  in  the  summer  and  warmer  in  the  winter  and  ex- 
ercises a  corresponding  effect  on  the  atmosphere  that  comes 
in  contact  with  it  Therefore  points  along  the  sea  coast 
have  a  more  uniform  climate  than  inland  points.  The 
same  thing  is  true  in  a  less  degree  of  the  lakes,  especially 
the  larger  lakes.  Even  the  Finger  lakes  of  central  New 
York  temper  the  climate  on  their  shores,  but  their  influence 
does  not  extend  so  far  away  as  is  the  case  with  larger 
bodies  of  water. 

The  chief  reasons  for  the  tempering  influence  of  the 
water  on  the  air  are  (1)  the  greater  specific  heat  of  water 
so  that  it  requires  about  twice  as  much  heat  to  raise  a 
given  volume  of  water  one  degree  as  it  does  a  similar  vol- 


THE    ATMOSPHERE  363 

ume  of  earth;  (2)  being  more  mobile  the  water  warmed 
in  one  place  is  carried  by  currents  to  another  and  colder 
part  of  the  ocean,  and  since  cold  water  is  denser  and 
heavier,  the  water  cooled  at  the  surface  sinks  to  the  bot- 
tom and  the  surface  is  not  frozen  until  the  whole  body  of 
water  is  reduced  to  the  point  of  greatest  density  which  is 
near  the  freezing  point;  hence  large  bodies  like  the  ocean 
are  never  frozen  in  our  latitude  but  the  deeper  portions 
are  always  cold ;  ( 3 )  part  of  the  heat  received  on  the  water 
is  expended  in  producing  evaporation  and  hence  does  not 
raise  the  temperature;  (4)  clouds  and  fog  which  check 
radiation  of  heat  are  more  common  over  water  than  over 
land  areas. 

Effect  of  Clouds. — Cloudiness  retards  loss  of  heat  by 
radiation.  In  the  spring  and  autumn  when  the  tempera- 
ture is  near  the  freezing  point  one  frequently  hears  the 
remark,  *'If  it  clears  off  to-night,  there  will  be  a  frost." 
On  a  clear  night  the  heat  received  from  the  sun  during 
the  day  is  rapidly  radiated  out  into  space.  If  the  clouds 
are  present  this  radiation  is  checked  in  a  large  degree,  be- 
cause they  do  not  permit  the  heat  to  pass  through  as  readily 
as  through  clear  atmosphere  and  they  also  radiate  and  re- 
flect heat  back  to  the  earth. 

Prevailing  winds  affect  the  temperature  in  some  places. 
In  India  for  about  half  the  year,  the  winds  blow  from  the 
Indian  Ocean  warm  and  moist.  During  the  other  half 
they'  come  from  the  Thibetan  Plateau  and  Himalaya 
Mountains  dry  and  cold. 

Influence  of  topography.  In  high  latitudes  where  the  sun's 
rays  are  quite  slanting  even  at  midday,  there  is  great  difference 
in  the  quantity  of  heat  received  on  the  warm  south  slopes,  which 
may  be  at  nearly  right  angles  to  the  noonday  sun,  from  that  on 
the  cool  north  slopes  where  the  rays  may  be  nearly  tangent  or 
in  some  cases  never  strike  even  at  noonday.  Slopes  facing  west 
or    southwest    are   warmer    than    those    facing    east   or    south- 


364 


PHYSICAL    GEOGRAPHY 


east.  The  difference  may  be  manifest  in  a  region  moderately 
hilly,  but  is  still  more  pronounced  in  a  mountainous  country, 
especially  so  where  the  mountains  extend  east  and  west.  Are 
there  any  spots  in  your  vicinity  where  the  spring  flowers  bloom 
first?  Where  the  early  fruits  first  ripen?  What  is  the  relation 
of  such  spots  to  the  sunshine?     (See  fig.  249.) 


Fig.  249.  Diagram  showing  different  effects  of  the  sunshine  on  the  two 
Bides  of  a  valley.  The  north  side  of  the  valley  receives  the  nearly 
vertical  rays  of  the  sun  at  noonday,  while  the  south  side  is  in  the 
shadow.  In  the  winter  season  this  causes  more  frequent  thawing  and 
freezing,  and  hence  more  rapid  weathering  on  the  north  side,  and  in 
the  spring  it  causes  earlier  vegetation. 

291.  Isotherms.— The  thermometer  is  read  and  re- 
corded twice  daily,  at  8  o'clock  morning  and  evening,  at  a 
great  many  stations  in  the  United  States  and  Canada  and 
the  results  distributed  by  telegraph  to  certain  places  where 
they  are  recorded  on  maps  and  lines  drawn  connecting 
points  having  the  same  temperature ;  such  lines  are  called 
isotherms,  meaning  equal  temperatures.  By  consulting  a 
number  of  daily  weather  maps  for  successive  days,  it  will 
be  seen  that  there  is  considerable  variation  in  the  position 
of  the  isotherms  from  day  to  day.  These  may  all  be 
averaged  ^nd  a  map  constructed  showing  the  mean  for 
the  month.  Fig.  250  shows  an  isothermal  map  for  July, 
one  for  January  and  another  for  the  year.  Compare  these 
carefully  and  account  for  the  different  positions  of  the 
same  isotherms. 


Fig.  250.  Isothermal  chart  for  January,  July  and  for  the  year. 
Note  how  the  isotherms  move  southward  in  the  northern  win- 
ter, and  north  in  the  summer.  Why?  Why  are  they  deflected 
so  much  more  in  North  America  in  the  summer  and  in  th© 
north  Atlantic  in  the   winter? 


366  PHYSICAL    GEOGRAPHY 

292.  Temperature  Gradient.— On  some  of  the  weather  maps, 
there  are  more  isotherms  than  on  others,  showing  at  times  a 
much  greater  difference  -or  range  of  temperature.  In  general, 
great  differences  in  temperature  are  found  associated  with  great 
differences  in  pressure;  in  fact  extremes  of  temperature  cause 
extremes  of  pressure.  The  difference  between  the  high  and  low 
temperatures  is  called  a  temperature  gradient  and  the  gradient 
is  higher  when  the  isotherms  are  most  numerous  and  closest  to- 
gether. Steep  temperature  gradients  are  associated  with  steep 
or  decided  pressure  gradients.  Compare  figs.  245  and  248  and 
note  that  high  pressure  is  associated  with  low  temperature  and 
vice  versa.    The  gradients  are  high  in  each  case. 

293.  Temperature  Zones.- In  the  isothermal  chart  of 
the  world,  it  is  shown  that  the  isotherms  of  70  degrees  lie 
some  distance  on  each  side  of  the  equator  but  at  different 
distances  in  the  January  and  in  the  July  charts.  These 
isotherms  inclose  the  warm  or  hot  zone  through  the  midst 
of  which  runs  a  line  of  highest  temperature,  the  heat 
equator,  which  lies  north  of  the  true  equator  in  July  and 
south  of  it  in  January;  that  is,  it  shifts  north  and  south 
following  the  sun.  The  temperature  belt  inclosed  between 
the  isotherms  of  70°  and  30°  is  called  the  temperate  zone, 
while  the  region  around  the  poles  outside  of  the  thirty  de- 
gree isotherm  has  a  frigid  temperature.  All  of  these  zones 
it  will  be  observed  shift  north  and  south  following  the 
movements  of  the  heat  equator.     (Study  fig.  251.) 

All  of  the  temperature  zones  are  more  nearly  uniform  in  the 
southern  hemisphere  than  in  the  northern,  and  more  uniform  on 
the  southern  oceans  than  on  the  southern  continents.  Why? 
The  student  should  be  able  to  state  the  reasons  from  the  data  in 
the   preceding   pages. 

MOVEMENTS    OF    THE    ATMOSPHERE 

294.  Winds,  Currents  and  Calms.—  The  direct  cause 
of  the  movements  of  the  air  is  difference  in  pressure,  which 
in  turn  is  due  to  difference  in  temperature.     Thus,  when 


THE    ATMOSPHERE 


367 


Fig.  251.  Climatic  zone  map  "based  on  isotherms.  Point  out  some  of  the  varia- 
tions from  a  zonal  map  based  on  the  tropical  and  polar  circles.  Compare 
with  Fig.  250  and  note  how  the  zones  may  shift  with  the  seasons. 


368  PHYSICAL    GEOGRAPHY 

the  air  at  any  place  becomes  heated  it  expands  and  is 
pushed  up  by  the  surrounding  air  crowding  in  to  take  its 
place,  which  in  turn  is  replaced  by  air  descending  at  some 
other  place.  So  winds  and  air  currents  are  produced. 
The  movements  more  or  less  horizontal,  where  rapid  enough 
to  be  perceptible,  are  called  winds.  The  vertical  move- 
ments, both  up  and  down,  are  not  ordinarily  perceptible 
and  are  known  as  calms.  The  equatorial  calms  are  formed 
by  rising  currents  of  air  and  the  tropical  calms  by  descend- 
ing currents,  while  the  movement  from  the  tropical  calms 
to  the  equatorial  calms  forms  the  trade  winds. 

Classification  of  the  winds.  The  winds  may  be  grouped 
for  classification  into  terrestrial  or  planetary,  cyclonic  or 
eddying,  and  continental.  The  wind  is  named  from  the 
point  of  the  compass  from  which  it  is  blowing,  hence  a 
w^est  wind  means  a  movement  of  the  air  from  the  west  to- 
ward the  east. 

295.  Terrestrial  or  planetary  winds  are  those  due  to 
the  condition  of  a  rotating  planet  heated  from  an  external 
source,  and  occur  on  all  planets  that  have  an  atmosphere. 
There  is  an  excessive  heating  in  the  region  of  the  equator, 
which  causes  the  air  to  expand  and  flow  off  aloft,  thus  pro- 
ducing a  low  pressure  belt  of  calms  known  as  the  doldrums 
or  the  equatorial  calms  and  a  high  pressure  near  the  tropics 
known  as  the  horse  latitudes  or  tropical  calms  formed  by 
descending  air. 

Trade  winds.  The  warm  air  in  the  doldrums  is  forced 
upwards  by  the  air  which  crowds  in  from  the  trade  winds 
on  both  sides.  The  trade  winds  do  not  move  directly  north 
and  south  to  the  equator  but  are  deflected  to  the  west  be- 
cause of  the  rotation  of  the  earth,  which  according  to  Fer- 
rel's  law  causes  a  deflection  of  all  winds,  cyclonic  as  well 
as  terrestrial,  to  their  right  in  the  northern  hemisphere 
and  to  their  left  in  the  southern  hemisphere.     It  is  be- 


V"  OF  THt 


of 


calii 


VARIABLE 


WINDS 


J.    W.    REDWAY 


THE    ATMOSPHEKE 


369 


cause  of  the  regularity  with  which  these  winds  blow  that 
they  are  called  trade  winds.  While  they  are  still  im- 
portant factors  in  commerce  they  were  much  more  so  when 
all  the  vessels  were  sailing  vessels  before  the  days  of 
steam  navigation. 


winds  and  Ralna  o(  July-^Motthftm  SottmoC? 

Fig.  253.     Planetary  wind  belts  in  summer  and  winter.     The  heat  equator  lies 
in  the  midst  of  the  equatorial  rain  belt. 
24 


370  PHYSICAL    GEOGRAPHY 

Since  the  doldrum  belt  shifts  north  and  south  following 
the  sun  during  the  change  of  seasons,  the  trade  wind  belts 
shift  with  it.     (See  fig.  253.) 

The  antitrades  are  caused  by  the  ascending  air  at  the 
equator  overflowing  out  toward  the  poles  in  the  higher 
atmosphere,  above  the  trade  winds  and  in  an  opposite  di- 
rection. In  the  vicinity  of  the  tropics,  the  antitrades 
descend  in  part  to  the  surface,  but  since  the  descent  is 
vertical  or  nearly  so,  they  form  belts  of  calms  known  as  the 
horse  latitudes  or  tropical  calms. 

The  prevailing  westerlies  are  the  winds  blowing  from 
the  horse  latitudes  towards  the  poles,  but  like  the  trade 
winds  and  all  other  atmospheric  movements,  they  follow 
Ferrel's  law  and  are  deflected  to  the  right  in  the  northern 
hemisphere  and  to  the  left  in  the  southern  hemisphere, 
thus  becoming  northwest  winds  in  the  southern  hemisphere 
and  southwest  winds  in  the  northern.  They  are  much  less 
regular  in  their  movement  than  the  trade  winds,  frequently 
shifting  direction  to  take  part  in  the  great  spiral  whirls 
known  as  cyclones  and  anticyclones.  They  are  also  sub- 
ject to  many  local  disturbances  such  as  land-and-sea  breezes 
and  the  mountain  and  valley  breezes.  Beyond  the  belt  of 
the  prevailing  westerlies,  the  movements  of  the  atmosphere 
are  not  so  well  known;  the  winds  are  thought  to  circle 
around  the  poles  forming  the  circumpolar  whirl  or  eddy. 

296.  Cyclonic  Winds.— A  second  class  of  winds  more 
local  and  variable  than  the  terrestrial  ones  are  the  cyclones, 
in  which  the  winds  move  inward  and  upward  in  a  spiral 
whirl  or  eddy  around  a  region  of  low  barometric  pressure. 
In  temperate  climates,  cyclones  occur  in  the  belt  of  the  pre- 
vailing westerlies  where  they  cover  large  areas,  sometimes 
1,000  miles  or  more  in  diameter. 

There  are  two  movements  of  the  air  in  a  cyclone,  a 
horizontal   one  towards  the  center  and  a  vertical  one  at 


THE    ATMOSPHERE 


371 


and  near  the  center.  The  origin  of  the  movement  is  prob- 
ably due  to  the  increase  in  temperature  at  the  center  which 
causes  the  air  to  expand  and  overflow  in  the  upper  atmos- 
phere, producing  a  downward  pressure  on  the  surrounding 


Fig.  254.  Cyclone  (low)  and  anticyclone  (high)  areas  in  United  States. 
Line  of  arrows  show  path  over  which  the  central  low  has  passed.  Shaded 
areas  indicate  where  rain  has  fallen  during  preceding  24  hours.  Dotted 
lines,  isotherms,  continuous  lines,  isobars.  Arrows  show  direction  of  wind. 
Black  circle  on  the  arrow,  cloudy,  open  circle,  fair  weather.  The  great 
storm  of  Feb.,  1902  which  caused  excessive  floods  in  the  Ohio  valley. 
(U.  S.  Weather  Bureau.) 


area.  The  crowding  towards  the  center  by  this  downward 
pressure  pushes  the  expanded  air  up  and  the  movement  is 
continued  until  equilibrium  is  again  restored.  The  area 
is  variously  designated  a  cyclone,  a  low  pressure  area  or  a 
low.  It  should  be  noted  that  the  violent  whirling  storms 
which  prove  so  destructive  to  life  and  property,  called 
cyclones  in  the  newspapers,  are  properly  called  tornadoes, 


372 


PHYSICAL    GEOGRAPHY 


and  are  described  below.  One  or  more  cyclones  pass 
across  the  United  States  nearly  every  week.  (Figs.  254 
and  255.) 

An  anticyclone  is  an  area  of  high  pressure  or  a  high, 
where  the  air  is  descending  and  the  winds  blow  out  along 
the  surface  from  the  center.  As  the  name  indicates  it  is 
the  opposite  of  a  cyclone,  the  winds  blowing  from  the  anti- 
cyclone or  high,  to  the  low.  The  higher  or  steeper  the 
pressure  gradient,  that  is,  the  greater  the  difference  be- 
tween the  pressure  in  the  center  of  the  low  and  the  high, 
the  stronger  will  be  the  winds.  The  air  flows  down  the 
pressure  slope  at  a  rate  proportional  to  the  steepness  of 
the  slope. 

The  weather  in  the  United  States  is  in  a  large  measure 
determined  by  the  * '  highs ' '  and  '  *  lows ' '  which  move  across 
the  country  in  the  general  direction  of  the  prevailing 
winds.     (See  sec.  319.) 


Fig.  255.  Mean  tracks  of  the  high  and  low  pressure  areas  across  the  United 
States  and  the  average  daily  movement  of  the  same.  (U.  S.  Weather 
Bureau.) 


THE    ATMOSPHERE 


373 


Movements  of  cyclones  in  the  United  States.  Cyclones  some- 
times enter  the  United  States  from  the  northwest  into  Montana 
or  Dakota  and  travel  southeast  to  near  the  middle  of  the  Missis- 
sippi valley  and  then  northeast  to  and  sometimes  across  the 
Atlantic  Ocean.  Sometimes  they  develop  in  the  southwest  in 
New  Mexico  and  Texas  and  then  move  northeast  off  the  con- 
tinent. Sometimes  they  come  in  from  the  Pacific  Coast.  Some- 
times they  pass  out  of  the  United  States  in  the  southeast.  The 
rate  of  advance  differs  somewhat,  but  is  generally  faster  in  the 
winter,  averaging  about  800  miles  per  day,  and  slower  in  the 
summer,  about  500  miles  per  day.  (See  fig.  255.)  Test  this  by 
actual  measurement  from  a  series  of  weather  maps  for  successive 
days. 

297.  Hurricanes. — The  tropical  cyclones  or  hurricanes 
are  great  whirling  storms  from  100  to  300  miles  in  dia- 
meter, in  which  the  winds  frequently  become  very  violent 


Fig.  256.     Map   showing   track   of   the    Galveston   hurricane.      From  the   dates 
the  rate  of  movement  can  be  estimated.      (U.  S.  Weather  Bureau.) 


374 


PHYSICAL    GEOGRAPHY 


and  destructive  near  the  center.  In  the  very  center,  how- 
ever, the  winds  die  away  leaving  a  calm  with  a  clear  sky, 
know^n  as  the  ' '  eye  of  the  storm ' '  which  is  said  to  vary  from 
10  to  20  miles  in  diameter,  while  immediately  around  it 
the  winds  are  most  violent,  decreasing  in  intensity  further 


Fig.  257.     Map  showing  track  ot   the  Galveston  hurricane  through  a  forest  in 
Texas.     The  trees  nearly  all  lie  parallel  with  each  other.      (W.  S.  Bray.) 

from  the  center.  The  hurricanes  originate  on  the  oceans 
within  the  tropics.  The  South  Atlantic  ocean,  however, 
appears  to  be  free  from  them.  In  the  North  Atlantic  they 
start  near  the  West  Indies  and  are  known  as  West  India 
hurricanes.  They  generally  move  in  a  northwesterly  direc- 
tion to  the  coast  of  Florida  and  thence  northeasterly  across 
the  Atlantic  occasionally  reaching  the  coast  of  Europe  be- 
fore they  are  dissipated.  Sometimes  they  move  up  the 
east  coast  of  the  United  States,  causing  great  destruction 


THE    ATMOSPHERE 


375 


to  the  shipping.  Occasionally  one  of  the  hurricanes  passes 
into  the  Gulf  of  Mexico  and  thence  into  the  Gulf  States. 
It  was  one  of  these  tropical  hurricanes  that  passed  over 


Pig.   258.     Photograph    of   a    tornado.      Note    the   funnel   shaped    cloud    around 
which  the  winds  move  with  great  velocity.      (W.  E.   Seright. ) 

the  city  of  Galveston  in  the  year  1900  and  destroyed  the 
greater  part  of  the  city,  besides  doing  much  damage  in  the 
country  farther  north.     Similar  storms  in  the  Pacific  Ocean 


376  PHYSICAL    GEOGRAPHY 

are  called  typhoons  and  often  prove  very  destructive  in 
the  region  of  the  Philippines  and  Japan.  (Figs.  256  and 
257.) 

298.  Tornadoes  are  cyclonic  whirlwinds  of  small  area 
and  great  intensity  that  originate  in  the  region  of  the  pre- 
vailing westerlies.  They  are  associated  with  thunder- 
storms in  the  summer  season  and  are  most  common  on 
the  plains  of  the  west  and  southwest.  It  is  not  often  that 
a  violent  tornado  occurs  east  of  the  Alleghany  Mountains, 
yet  it  does  sometimes. 

They  are  generally  marked  by  a  dark  funnel-shaped 
cloud  suspended  from  the  black  mass  of  the  thunder  cloud 
(see  fig.  258).  The  storm  generally  moves  east  or  north- 
east at  a  rate  varying  from  20  to  40  miles  an  hour  but  the 
rotary  velocity  of  the  wind  in  the  whirl  may  reach  500 
miles  or  more.  It  is  the  most  violent  class  of  storms  known 
in  the  United  States  and  some  of  the  effects  produced  are 
almost  incredible,  such,  for  instance,  as  plucking  the  feath- 
ers from  a  chicken,  tearing  the  tires  from  a  wagon,  tearing 
the  lath  from  a  house  and  driving  them  through  the  roof 
of  a  barn. 

When  a  tornado  occurs  on  a  lake  or  the  ocean,  a  column  of 
water  is  often  formed  in  the  vortex  and  it  is  then  called  a  water- 
spout. A  vessel  caught  in  one  of  these  waterspouts  is  liable  to 
severe  injury  if  not  total  destruction.  It  is  thought  that  the 
water  in  the  waterspoi^t  is  mostly  condensed  from  the  clouds 
rather  than  drawn  up  from  the  sea. 

299.  Hot  Waves.- The  warm,  south  winds  drawn 
northward  into  a  low  pressure  area,  when  unseasonably 
warm  and  dry,  are  called  siroccos.  The  typical  siroccos 
occur  in  Italy  and  are  caused  by  the  hot,  scorching  winds 
of  the  African  desert  flowing  towards  a  low  pressure  area 
in  central  Europe.  Similar  but  not  such  strongly  marked 
siroccos  occur  at  times  in  the  Mississippi  Valley,  where 


THE    ATMOSPHERE  377 

they  are  known  as  hot  waves  and  cause  drouth  in  the  sum- 
mer season  and  thaws  in  the  winter.  In  Australia  such 
winds  are  known  as  h  rick  fielders. 

The  chinooTc  is  the  warm,  drying  wind  that  descends  the  east- 
ern slope  of  the  Rocky  Mountains  to  the  great  plains.  The 
moisture  has  been  precipitated  on  the  west  slope  and  summit  of 
the  mountains  and  the  air  descends  on  the  plains  as  dry,  hot 
winds. 

300.  Cold  Waves.— A  cold  wave  signifies  a  sudden 
fall  of  the  thermometer  resulting  in  temperatures  extreme- 
ly low  for  the  season  in  any  given  locality.  For  the  winter 
season  in  central  New  York  a  cold  wave  is  defined  as  a  24 
hour  temperature  fall  of  20  degrees  or  more  to  a  minimum 
of  10  degrees  or  lower,  while  in  the  warmest  portions,  of 
the  United  States  a  fall  of  16  degrees  to  a  minimum  of  32 
degrees  only  is  required.  It  follows  a  cyclone,  precedes 
an  anticyclone,  and  is  produced  by  the  cold  winds  from 
the  plains  of  the  west  and  northwest  moving  towards  a  low 
to  the  east.  It  is  commonly  accompanied  by  fair  weather 
but  occasionally  there  is  a  fine  drifting  snow  and  high 
winds  forming  the  much  dreaded  blizzard  of  the  western 
plains.  The  Norther  of  Texas,  the  huran  of  Siberia  and 
the  northeaster  of  western  Europe  are  local  names  for  a 
cold  wave  in  the  different  countries.  The  last  is  produced 
when  a  low  pressure  swings  a  little  farther  south  than 
usual  and  the  cold  winds  are  drawn  down  from  the  plains 
of  northern  Europe.  The  northeaster  in  the  United  States 
is  much  dreaded  along  the  Atlantic  coast  as  it  is  frequently 
the  border  of  an  advancing  hurricane  from  the  south, 
which  means  danger  to  coasting  vessels. 

301.  Continental  Winds.— A  third  class  of  winds, 
caused  by  local  differences  in  the  rate  of  radiation  and  ab- 
sorption over  land  and  water  areas,  is  called  continental 
winds,  the  most  marked  type  of  which  is  the  monsoon. 


378  PHYSICAL    GEOGRAPHY 

302.  Monsoons.-  The  monsoons  are  best  developed  in 
India  where  the  sea  breeze  in  the  summer  is  so  strong  as  to 
reverse  the  northern  trade  winds  and  cause  the  southeast 
trades  to  continue  across  the  equator  and  over  India  as 
southwest  winds.  In  the  passage  across  the  tropical  seas, 
the  air  is  heavily  charged  with  moisture,  which  is  precipi- 
tated on  the  south  slopes  of  the  Himalayas,  producing  an 
enormously  heavy  rainfall,  in  places  as  high  as  35  feet  per 
year.  In  the  winter  the  winds  are  reversed,  the  cold  winds 
from  the  plateau  of  central  Asia  blowing  across  India  to 
the  sea.  The  winds,  warming  as  they  descend  the  moun- 
tains, blow  across  India  as  dry  winds,  taking  up  instead 
of  precipitating  moisture  and  when  they  are  prolonged  they 
produce  drouth  and  famine  in  the  land.  The  monsoons 
are  caused  by  unequal  heating  of  land  and  water,  the  land 
being  warmer  than  water  in  the  summer  and  cooler  in 
winter.      (See  fig.  252.) 

303.  Land  and  Sea  Breezes.- The  daily  changes  be- 
tween land  and  sea  are  similar  but  less  pronounced  than 
the  seasonal.  The  land  is  heated  during  the  day,  causing 
the  air  to  expand;  the  inflow  from  the  sea  produces  the 
sea  hreeze  during  the  middle  of  the  day  but  dies  out  in  the 
night.  At  night  the  land  cools  more  rapidly  and  the 
wind  is  reversed  to  the  land  hreeze  blowing  from  the  land 
to  the  sea  during  the  night  and  in  the  early  morning.  This 
reversal  of  the  winds  is  utilized  by  the  fishermen  who  sail 
out  on  the  land  breeze  in  the  early  morning  and  return  on 
the  sea  breeze  in  the  evening. 

304.  Mountain  and  Valley  Breezes  have  a  similar  origin.  The 
mountain  radiates  heat  more  rapidly  at  night  than  the  valley, 
hence  the  heavy  and  cool  air  (heavy  because  cool)  flows  down 
the  mountain  and  down  the  valley  forming  the  mountain  breezes. 
In  the  daytime  the  reverse  takes  place  when  the  valley  hreeze 
blows  up  the  valley  and  up  the  mountain.     Th«  mountain  breezes 


THE    ATMOSPHERE 


379 


are  generally  stronger  than  the  valley  breezes  because  in  blowing 
down  the  slopes  they  are  aided  by  gravity. 
305.  Wind  Velocity.— The  velocity  of 
the  wind  is  recorded  by  an  anemometer 
(anemo,  wind;  meter,  measure).  The 
style  used  by  the  Weather  Bureau  is 
shown  by  the  accompaning  figure.  The 
wind  blowing  into  the  cups  causes  them 
to  revolve  rapidly  about  the  vertical  axis, 
the  rate  of  movement  being  indicated  in 
miles  per  hour  by  an  index  at  the  base  of 
the  standard.  The  winds  are  roughly 
classified  according  to  velocity  as  follows : 

1.  Calm    signifies    no    movement    or 
less  than  one  mile  per  hour. 

2.  Light  wind,  less  than  10  miles  per 
hour,  moves  leaves  on  trees. 

3.  Moderate,  10  to  15  miles  per  hour,  moves  small  branches. 

4.  Brisk,   15    to   25   miles,   sways   branches,   raises   dust. 

5.  High  wind,  25  to  40,  sways  trees. 

6.  Gale,  40  to  60,  breaks  branches,  uproots  trees. 

7.  Hurricane  and  tornado,  above  60,  sometimes  500  miles  per 
houj",  destroys  houses. 

The  direction  of  the  wind  is  indicated  by  a  ivind  vane  which 
consists  of  an  arrow  with  two  broad  divergent  flanges  on  the 
opposite  end,  free  to  rotate  on  a  vertical  axis.  The  arrow  points 
in  the  direction  from  which  the  wind  is  blowing.  The  arrows  on 
the  weather  maps  point  the  direction  in  which  the  wind  is 
blowing. 


Fig,  259.  An  anemometer 
or  wind  gauge  for  meas- 
uring the  velocity  of  the 
wind. 


HUMIDITY  AND  PRECIPITATION 


306.  Absolute  and  Relative  Humidity.— The  atmos- 
pliere  always  carries  some  moisture  in  the  form  of  invisible 
water  vapor  which  is  obtained  by  its  contact  with  the  sur- 
face of  the  ocean  and  the  moist  land.  The  amount  of 
moisture  in  the  atmosphere  varies  at  ^different  places,  and 
at  the  same  place  at  different  times.  The  quantity  of 
water  in  a  given  volume  of  air  at  any  time  expressed  in 


380  PHYSICAL    GEOGRAPHY 

grains  per  cubic  foot  denotes  its  absolute  humidity.  The 
amoimt  of  vapor  present  in  the  air,  compared  with  wHat 
might  be  present  if  the  air  were  saturated  with  moisture, 
gives  the  relative  humidity  and  is  expressed  in  per  cent. 
If  the  air  were  perfectly  free  from  moisture,  which  it  never 
is,  the  relative  humidity  would  be  zero;  when  it  is  satur- 
ated, the  relative  humidity  is  100  per  cent. 

While  the  absolute  humidity  may  remain  constant,  the  rela- 
tive humidity  varies  with  the  temperature.  The  capacity  of  the 
air  for  moisture  increases  with  an  increase  in  temperature. 
Thus  the  air  may  be  saturated,  the  relative  humidity  100  per 
cent  at  one  temperature,  say  60  degrees,  and  if  the  temperature 
be  raised  to  80  degrees  the  relative  humidity  will  fall  consider- 
ably below  100.  On  the  other  hand,  should  the  temperature  be 
lowered  when  the  relative  humidity  is  100,  precipitation  will  take 
place,  that  is,  rain  or  snow  will  fall.  When  you  hear  the  expres- 
sions, "The  air  is  raw,"  "It  is  penetrating,"  what  can  you  infer 
concerning  the  humidity?  Why  does  cold,  moist  air  feel  colder 
and  warm  moist  air  warmer  than  dry  air  at  the  same  tem- 
peratures? 

307.  Dew  Point.— The  temperature  at  the  point  of 
saturation  is  known  as  the  dew  point,  and  may  be  deter- 
mined experimentally  by  placing  some  ice  in  a  cup  of 
water  and  stirring  it  with  a  thermometer  until  moisture 
begins  to  form  on  the  outside  of  the  cup.  The  reading  of 
the  thermometer  at  that  time  will  be  the  dew  point  of  the 
atmosphere  at  that  instant.  By  trying  this  experiment  at 
several  different  times,  it  may  be  noticed  that  this  varies 
considerably  in  air  even  at  the  same  temperature. 

308.  Instruments.— There  are  several  different  instru- 
ments used  for  measuring  the  humidity  of  the  air.  The 
essential  part  of  a  common  hygrometer  consists  of  human 
hair  deprived  of  its  oil,  which  changes  in  length  with  the 
percentage  of  moisture  in  the  air.  It  is  called  the  hair 
hygrometer. 

The  sling  psychrometer  consists  of  two  standard  ther- 


THE    ATMOSPHERE  .  381 

mometers  attached  to  a  board,  one  of  which  has  the  bulb 
covered  with  wet  muslin.  They  are  whirled  through  the 
air  for  a  short  time  to 
hasten  the  evaporation 
from  the  muslin.  If  the 
air  is  saturated  with 
moisture,  the  two  ther- 
mometers will  read  the  '^^^ 
same,  but  if  the  relative 
humidity  is  low,  there 

(will    be    rapid    evapora-  Fig.   2b0.      Hygrometer.       An     instrument     for 
tion     from     the     muslin  ^^^"^f"^   the  relative  humidity  of  the  at- 

mosphere.       A   wet   and   dry   thermometer 
covering   the   wet   bulb,  is    commonly   used    and   the    results    com- 

causing  the  mercury  to        pared. 

fall.    The  difference  between  the  wet  and  dry  bulb  readings 

will  increase  as  the  relative  humidity  decreases. 

A  Tiygrodeik  is  a  form  of  hygrometer  in  which  the  result  is 
shown  directly  by  adjusting  two  sliding  pieces  to  the  height  of 
the  mercury  in  the  wet  and  the  dry  bulb  thermometers,  in  such 
a  way  that  they  control  the  position  of  an  index  which  points 
out  the  number  indicating  the  relative  humidity  in  per  cent. 

309.  Dew  and  Frost.— When  rapid  radiation  from 
objects  on  the  surface  of  the  earth  causes  the  temperature 
of  the  air  in  contact  to  be  lowered  to  the  point  of  satura- 
tion, the  moisture  begins  to  condense,  the  point  of  satura- 
tion being  commonly  known  as  the  dew  point.  Dew  is 
formed  on  a  clear  night  by  the  rapid  radiation  of  the  heat 
from  the  surface  after  the  sun  goes  down.  The  air  com- 
ing in  contact  with  the  cooled  and  cooling  surface  is  chilled 
by  conduction,  when  some  of  the  moisture  condenses  as 
dew ;  or,  if  below  32  degrees  F.,  it  condenses  in  the  crystal 
form  as  lioar  frost.  Dew  or  frost,  is  formed  most  rapidly 
on  the  surface  of  substances  which  are  the  best  radiators  of 
heat,  such  as  stone,  grass  and  leaves. 


382 


PHYSICAL    GEOGRAPHY 


Less  dew  is  formed  on  a  cloudy  than  on  a  clear  night  because 
the  clouds  check  radiation  and  prevent  the  surface  from  being 
sufficiently  cooled.  Dew  is  not  formed  on  a  windy  night,  because 
the  air  does  not  remain  long  enough  in  contact  with  the  cool 
surface  to  be  lowered  to  the  dew  point. 

310.  Clouds.—  The  condensation  of  the  moisture  in  the 
air  produces  clouds  of  many  different  shapes  and  sizes. 
A  cloud  at  the  surface  of  the  earth  is  called  fog,  or,  if  very 
light,  mist.  In  the  fog  or  cloud,  the  moisture  has  been  suf- 
ficiently condensed  to  form  small  particles  large  enough  to 
intercept  the  rays  of  light. 

311.  Classification  of  Clouds.— The  more  common 
forms  of  clouds  are:  (1)  the  cumulus  which  often  resem- 


FlG.   261      Cumulus    cloud. 
cedes  a  thunder  storm. 


Common  in  the  sumimsr   scasou.     IVequeutly   pru 


bles  great  masses  of  snowy  wool  or  cotton.  They  com- 
monly have  a  flat  or  nearly  regular  base  but  a  very  irre- 
gular and  changing  top  and  are  among  the  most  common 
cloud  forms  in  the  summer  season.  They  are  formed  by 
the  ascending  currents  of  warm  air  from  the  heated  land 
surface.     They  are  'also  called  thunderheads  and  commonly 


THE    ATMOSPHERE  383 

precede   a   thunderstorm.     The  base   is   generally   half   a 
mile  to  a  mile  above  the,  surface  of  the  earth. 


Fig.  262.     Plumed  cirrus  cloud.     Height  8000  meters  in  winter,  9700  meters 
summer.      (E.  E.  Howell.) 


384 


PHYSICAL    GEOGRAPHY 


(2)  The  cirrus  cloud  is  a  feathery,  plume-like  form 
that  occurs  at  a  height  of  five  to  ten  miles  above  the  surface 
and  often  consists  of  fine  ice  or  snow  crystals,  owing  to  its 
great  height.  It  forms  in  the  front  of  an  advancing  cy- 
clone or  low  pressure  area  and,  moving  ahead  with  the 
*'low,"  is  a  pretty  good  indication  of  the  advance  of  a 
storm  center.  It  has  been  called  a  ' '  weather-breedellr "  be- 
cause it  is  frequently  followed  by  rain  or  snow. 

(3)  Stratus  clouds  occur  in  layers  or  strata  near  the 
surface  and  frequently  accompany  rainstorms.  They 
sometimes  fall  to  the  surface  and  form  fogs.  They  are 
common  in  the  early  morning,  but  may  occur  at  other 
hours. 


Tic.  263.     Cirro    cumulus    cloud.      Height    6500    meters    summer.       (E. 
HoweU.) 


(4)  Nimbus  is  a  rain  cloud,  consisting  of  a  dark  grey 
to  black  mass  generally  covering  the  whole  sky  and  from 
which  the  rain  falls.     (The  term   ^^nimhus^'  refers  to  a 


THE    ATMOSPHERE  385 

condition  rather  than  to  a  form  of  cloud  and  is  usually 
understood  to  be  any  mass  of  cloud  from  which  rain  or 
snow  is  falling.)  Any  of  the  other  clouds,  especially  the 
stratus  or  cumulus,  may  rapidly  change  to  a  nimbus.  This 
is  the  most  common  cloud  form  in  New  York  State  during 
the  winter,  lasting  at  times  for  several  weeks  with  the  rain 
falling  at  intervals! 

The  different  cloud  forms  mentioned  may  form  many 
combinations,  as  cirro-cumulus j  cirro-stratus,  cumulo-stra- 
tus,  strato-cumulus. 

312.  Precipitation.— i?am  occurs  when  the  moisture 
condenses  into  drops  which  fail  to  the  earth.  If  the 
condensation  takes  place  at  temperature  below  32  degrees 
F.  it  forms  snow,  which  bears  the  same  relation  to  the  rain 
in  the  clouds  that  frost  does  to  dew  on  the  surface  of  the 
earth.  The  moisture  may  condense  as  snow  in  the  higher 
air  and,  in  falling  through  warmer  currents  near  the  sur- 
face, may  melt  and  reach  the  surface  as  rain.  But  if  the 
rain  should  freeze  while  falling  through  the  lower  air  it 
would  not  form  snow  but  sleet.  Sleet  may  also  be  half- 
melted  snow. 

Hail  is  thought  to  be  a  mixture  of  snow  and  frozen  rain. 
It  is  formed  during  thunder- 
storms in  the  summer  season 
probably  by  the  passage  of 
the  descending  moisture  through 
several  air  currents  with  tem- 
peratures alternately  above  and 
below  the  freezing  point.  Hail 
storms,  coming  as  they  do  in  the 
summer  season  often  cause  great 
damage  to  vegetation. 

o-tn         ^         M.M  A     w«    .  m..       -ciw.    i4D4.        iippzng      ram      gauge. 

313.  Ouantitv    of    Rain. — The         >r  ^-^   &  ^        * 

vs"«'"*'-i«'j'     "A     ivrt'xxi.      xut;  Measures    and    records     auto- 

amount    of    rainfall    is    determined         maticaily  the  rainfall. 
25 


386 


PHYSICAL    GEOGRAPHY 


by  measuring  the  depth  of  water  in  a  vessel  known  as  a  rain- 
gauge.  A  section  of  the  rain-gauge  used  by  the  U.  S.  Weather 
Bureau  is  shown  in  fig.  264.  Snow  and  hail  are  melted  and  the 
result  given  in  the  amount  of  water  as  though  it  had  fallen  as 
rain.  It  takes  about  8  or  10  inches  of  snow  to  equal  an  inch  of 
rain,  but  this  differs  with  the  kind  of  snow. 

Most  all  the  rainfall  may  be  included  under  the  three 
heads:  cyclonic,  tropical  and  monsoon. 

314.  Cyclonic  Rains.— In  the  region  of  the  prevail- 
ing westerlies,  most  of  the  rainfall  comes  from  the  cyclonic 
or  low  pressure  areas.  As  the  cyclone  moves  east  across 
the  country,  the  warm,  moist  winds,  drawn  in  from  the 
south  and  east,  ascend  in  the  atmospheric  whirl  and  are 


189°       125°      121°       n7«      iiy     iQg"      106°      ipi"      »r       93°       88°       g5°       81°        77°        73°        W         06° 


Fig.  265.  Rainfall  map  of  the  United  States  showing  the  mean  annual  rainfall 
in  different  portions  of  the  country.  Give  your  reasons  for  the  great  vari- 
ation. More  of  the  zonal  lines  run  north  and  south  than  east  and  west. 
Account  for  the  maximum  and  minimum  in  the  same  latitude.  (U.  S. 
Weather  Bureau.) 


cooled  as  they  rise;  the  moisture  they  carry  is  condensed 
and  falls  as  rain  or  snow.     The  greater  part  of  the  rain 


THE    ATMOSPHERE  .  387 

falls  to  the  east  or  the  southeast  of  the  cyclone  center. 
Verify  this  by  study  of  the  weather  maps. 

Thunderstorms.  In  the  summer  season  the  cyclones 
are  frequently  accompanied  by  thunderstorms  which  are 
most  frequent  to  the  east  and  south  of  the  cyclone  center, 
but  are  not  limited  to  these  parts.  They  are  produced  by 
rapidly  ascending  warm  air  currents  which  produce  a 
heavy  cumulus  cloud,  the  downward  pressure  of  which 
causes  reversed  air  currents  to  spread  out  on  the  surface 
in  the  midst  of  the  ascending  warm  currents  in  front  of 
the  rapidly  moving  clouds.  The  outrushing  blast  of  cool, 
refreshing  air  is  generally  followed  closely  by  a  downpour 
of  rain  which  may  continue  for  a  few  minutes  or  for  sev- 
eral hours.  Thunderstorms  are  most  frequent  in  the  latter 
part  of  the  afternoon  or  night.  The  lightning  is  caused 
by  the  passage  of  the  electric  spark  from  cloud  to  cloud  or 
between  the  earth  and  the  cloud.  Thunder  is  the  sound 
caused  by  the  violent  agitation  of  the  air  along  the  flash. 
Since  the  velocity  of  light  is  nearly  instantaneous  for 
short  distances  and  sound  travels  about  twelve  miles  per 
minute,  the  distance  of  a  lightning  flash  may  be  roughly 
estimated  in  miles  by  dividing  by  five  the  number  of  sec- 
onds that  elapse  between  the  flash  of  lightning  and  the 
sound  of  the  thunder.  Much  of  the  rain  in  the  summer 
season  in  the  Mississippi  Valley  comes  from  the  thunder- 
storms. 

Cloudbursts  associated  with  thunderstorms  and  torna- 
does are  thought  to  be  caused  by  ascending  air  currents  so 
violent  that  they  hold  up  the  condensed  moisture  for  some 
time,  until  the  accumulation  Anally  breaks  through  and 
the  water  falls  in  a  mass  or  sheet,  frequently  causing  dis- 
aster on  the  surface  where  it  falls.  Cloudbursts  are  most 
frequent  in  dry  or  semi-arid  regions,  often  proving  destruc- 
tive in  the  mountains  of  Arizona,  New  Mexico  and  Col- 


388  ,      PHYSICAL    GEOGRAPHY 

orado.  Many  persons  have  been  drowned  from  the  cloud- 
bursts in  these  states  and  on  the  Sahara  and  other  deserts. 
The  tropical  rains  in  the  doldrum  belt  are  of  almost 
daily  occurrence.  The  clouds  begin  to  form  near  the  mid- 
dle of  the  day  and  heavy  rains,  generally  accompanied  by 
thunderstorms,  follow  in  the  early  afternoon.  The  sky 
clears  at  night  and  the  morning  is  fair.  These  rains  con- 
tinue throughout  the  year  shifting  north  and  south,  fol- 
lowing the  movements  of  the  heat  equator.     (See  fig.  253.) 

WEATHER  AND   CLIMATE 

315.  Weather  refers  to  all  the  atmospheric  conditions 
that  can  be  seen  or  felt,  such  as  (1)  the  temperature, 
whether  hot  or  cold,  or  growing  warmer  or  colder ;  ( 2 )  pre- 
cipitation, whether  rain  or  snow  and  how  much ;  ( 3 )  Cloud- 
iness, whether  fair,  partly  or  wholly  cloudy,  and  the  rela- 
tive humidity  of  the  air;  (4)  winds,  direction  and  velocity, 
and  changes. 

316.  Climate  is  the  sum  total  of  the  average  weather 
conditions  for  a  series  of  years;  its  consideration  should 
include  also  a  statement  of  the  extremes  or  variations  from 
the  normal,  and  would  have  the  same  elements  of  temper- 
ature, moisture  and  winds  as  the  weather.  The  whole  area 
of  the  earth  is  sometimes  divided  into  five  climatic  zones, 
separated  by  certain  parallels  of  latitude,  but  if  all  the 
elements  of  climate  are  considered,  the  zones  would  be  more 
irregular  and  might  have  a  number  of  subdivisions  such  as 
those  indicated  in  fig.  251. 

The  controlling  factors  of  the  climate  are: 
(1)  Latitude.  Outside  of  the  tropics  the  sun's  rays 
become  more  and  more  inclined  as  one  advances  towards 
the  poles  and  the  climate  becomes  correspondingly  cooler, 
hence  the  division  into  torrid,  temperate  and  frigid  cli- 
mates. 


THE    ATMOSPHERE  389 

(2)  Altitude.  Since  the  density  of  the  air  and  hence 
the  temperature  decreases  with  the  altitude,  cooler  climates 
will  be  found  in  ascending  the  mountains  and  plateaus. 

(3)  Distance  from  the  ocean  or  other  large  body  of 
water  affects  the  uniformity  of  temperature  and  frequently 
the  moisture  or  precipitation. 

317.  Effect  of  Mountains  on  Climate.— The  relation  of  moun- 
tains to  an  area  often  has  a  marked  effect  on  its  climate,  an 
effect  most  pronounced  in  the  region  of  prevailing  winds,  where 
they  blow  across  the  mountains.  In  the  trade  wind  belt  in  South' 
America,  there  is  a  heavy  rainfall  on  the  east  or  windward  side 
of  the  Andes  Mountains,  while  on  the  lee  side  there  is  a  very 
light  rainfall.  In  southern  India  in  the  monsoon  belt,  there  are 
heavy  rains  during  the  part  of  the  year  when  the  summer  mon- 
soons are  blowing,  while  the  remainder  of  the  year  is  dry. 

318.  The  terrestrial  wind  belts  are  factors  of  prime  impor- 
tance in  determining  the  climate.  The  doldrum  belt  has  its  al- 
most daily  rains  and  uniformly  moist  climate.  The  trade  wind 
belt  has  a  generally  uniform  fair  weather  on  the  seas  and  fre- 
quent rains  on  the  lands,  where  the  winds  blow  over  considerable 
elevations,  but  dry  on  the  lee  side  of  the  mountains.  Most  of  the 
deserts  occur  in  this  belt.  The  subtropical  belt  over  which  the 
horse  latitudes  migrate  have  prevailingly  fair  weather,  but  an 
area  lying  in  this  belt  is  swept  by  the  trade  winds  in  one  season 
and  by  the  prevailing  westerlies  at  another.  It  is  an  area  of  dry 
summers  and  rainy  winters.     (See  fig.  251.) 

319.  The  cyclone  and  anticyclone  paths  are  controlling 
factors  of  the  weather  in  the  belt  of  the  prevailing  wester- 
lies, and  determine  the  weather  changes  from  day  to  day. 
These  should  be  studied  carefully  from  the  daily  weather 
maps  published  by  the  government,  and  should  be  studied 
on  groups  of  maps  for  several  different  months  in  differ- 
ent seasons. 

The  following  weather  conditions*  may  be  looked  for 
in  association  with  the  lows  and  highs  in  the  United  States : 

"High  winds  with  rain  or  snow  usually  precede  the  low.  In 
advance  of  the  low  the  winds  are  generally  southerly  and  con- 
*  Quoted  from  the  Chief  of  the  Weather  Bureau. 


390  PHYSICAL    GEOGRAPHY 

sequently  bring  high  temperatures.  When  the  center  of  the  low 
passes  to  the  east  of  a  place,  the  wind  at  once  shifts  to  the  west 
or  northwest,  bringing  low  temperature.  The  temperature  on  a 
given  parallel  west  of  a  low  may  be  reasonably  looked  for  on  the 
same  parallel  to  the  east  when  the  low  has  passed.  Frost  will 
occur  along  the  north  of  an  isotherm  of  about  40  if  the  night  is 
clear  and  there  is  little  wind.  Following  the  low  usually  comes 
an  area  of  high,  bringing  sunshiny  weather,  which  in  turn  is  fol- 
lowed by  anoth  r  low. 

"The  cloud  and  rain  area  in  front  of  a  low  is  generally  about 
the  size  of  the  latter  and  oval,  with  the  west  side  touching  the 
center  of  the  low  in  advance  of  which  it  progresses. 

"When  the  isotherms  run  nearly  east  and  west  no  decided 
changes  in  temperature  will  occur.  If  the  isotherms  directly  west 
of  a  place  incline  northwest  to  southeast  it  will  be  warmer; 
if  from  northeast  to  southwest  it  will  be  colder. 

"An  absence  of  decided  waves  of  high  or  troughs  of  low 
pressure  indicates  a  continuance  of  existing  weather,  which  will 
last  until  later  maps  show  change,  usually  first  appearing  in  the 
west." 

320.  Weather  Maps.— At  eight*  o'clock  each  morn- 
ing and  evening  at  many  places  in  the  United  States, 
Canada,  Mexico  and  the  West  Indies,  observations  are 
made  on  the  weather  conditions.  A  record  is  made  of 
the  barometric  pressure,  temperature,  velocity  and  direc- 
tion of  the  wind,  condition  of  the  sky,  relative  humidity, 
and  amount  of  precipitation,  and  within  the  hour  these 
data  in  a  condensed  cipher  dispatch  are  sent  by  telegraph 
to  the  Weather  Bureau  in  Washington.  The  data  are 
rapidly  tabulated  and  transferred  by  appropriate  symbols 
to  a  weather  map,  which  is  in  turn  engraved  and  a  large 
edition  printed  and  delivered  to  the  mails  all  within  a 
remarkably  short  period  of  time.  Smaller  editions  of  a 
less  elaborate  map  are  printed  at  the  local  stations  in  dif- 
ferent cities  outside  of  Washington. 

*Eight  o'clock  by  the  75th  meridian  time,  which  means  7  o'clock  at  St. 
Louis,   6  o'clock  at  Denver,   and  5   o'clock  at   San  Francisco. 


THE    ATMOSPHERE  391 

The  data  shown  on  the  weather  map  consist  of  (1)  iso- 
bars, represented  by  solid  black  lines  drawn  through  points 
having  the  !l  ame  atmospheric  pressure,  a  line  for  each  tenth 
of  an  inch  on  the  barometer;  these  lines  curve  around  and 
enclose  the  lows  and  the  highs.  (2)  red  lines  (on  the  local 
maps,  dotted  lines)  are  drawn  for  the  isotherms,  one  for 
each  10  degrees  difference  in  temperature.  (3)  Heavy, 
broken  red  lines  enclose  areas  where  there  has  been  a  de- 
cided change  in  temperature  equal  to  a  rise  or  fall  of  20 
degrees  or  more  in  24  hours.  (4)  Shaded  areas  (on  the 
"Washington  map  but  not  shown  on  the  local  map)  indi- 
cate the  area  over  which  there  has  been  rain  or  snow  dur- 
ing the  past  24  hours.  (5)  The  condition  of  the  sky  is 
indicated  by  a  small  circle,  which  is  black  for  cloudy  sky 
and  open  for  clear  sky.  The  arrow  on  the  circle  indicates 
the  direction  of  the  wind.     R  signifies  rain  and  S  snow. 

Besides  the  graphic  representation  by  lines  and  sym- 
bols, all  the  data  are  printed  in  tabulated  form  on  the 
margin  of  the  map.  Daily  weather  maps  from  the  nearest 
Weather  Bureau  office  or  from  Washington  can  generally 
be  secured  on  application  and  should  be  studied  along  with 
the  text. 

Many  thousands  of  the  weather  ma^^o  are  distributed  daily 
through  the  mails  and  by  messengers.  Besides  the  maps  there 
are  thousands  of  cards  sent  out  from  the  local  stations  which 
contain  simply  the  weather  forecasts  for  the  next  24  hours. 
These  cards  are  sometimes  distributed  in  large  numbers  by  busi- 
ness firms  and  are  displayed  in  stores,  post  offices,  railway  sta- 
tions, elevators,  and  other  public  places.  In  some  places  the 
rural  mail  carriers  display  weather  signals  on  the  mail  carts  and 
in  places  the  signals  are  displayed  on  some  prominent  point  as 
a  steeple,  flagpole,  or  some  tall  building. 

The  flag  signals  are  as  follows:  A  square  white  flag  for  fair 
weather;  a  square  blue  flag  for  rain  or  snow;  a  triangular  black 
flag  above  the  white  or  blue  flag  indicates  followed  by  warmer 
weather;   if  below,  by  colder;   a  square  white  flag  with  a  square 


Fig.  266.  Daily  weather  map.  Note  the  movements  of  the  lows  and  highs 
for  the  two  days.  On  February  3rd,  the  intervening  day,  the  one  low 
was  central  over  southern  Maine,  the  other  over  the  Pan  Handle  of  Texas. 
(U.  S.  Weather  Bureau.) 


THE    ATMOSPHERE  393 

black  center  indicates  a  cold  wave  coming.     There  is  another  set 
of  flag  signals  in  use  for  wind  storms  on  the  lakes  or  sea  shore. 

321.  Benefits  from  Weather  Forecasts.— Some  of  the 
many  benefits  that  may  be  derived  from  the  widespread 
distribution  and  heralding  of  the  weather  forecasts  are  sug- 
gested in  the  following :  Knowledge  of  a  tropical  hurricane 
in  the  West  Indies  arrives  by  cable  and  storm  signals  are 
placed  in  all  the  harbors  along  the  Atlantic  coast  from  24 
to  36  hours  ahead  of  its  arrival,  by  which  many  vessels  are 
saved  from  destruction.  Similar  forecasts  of  storms  save 
a  great  many  boats  on  the  Great  Lakes.  Some  of  the  in- 
surance companies  recognize  the  value  of  this  branch  of 
the  service  by  refusing  all  risks  on  vessels  that  go  out 
against  the  warnings. 

The  news  of  a  decided  cold  wave  coming  from  the  northwest 
causes  quite  a  flutter  in  many  lines  of  business,  the  ice  com- 
panies, the  coal  dealers,  the  railway  employees  in  charge  of 
perishable  goods,  the  fruit  commission  merchants,  stock  raisers, 
and  many  others  who  take  such  precautions  as  they  can  to  pre- 
vent loss.  An  important  branch  of  the  service  consists  in  the 
warnings  of  floods  along  the  larger  rivers  in  which  the  fore- 
knowledge is  often  the  means  of  saving  a  great  deal  of  property. 

The  student  may  enumerate  other  ways  in  which  benefit  may 
be  derived  from  the  foreknowledge  of  the  weather  changes. 

The  weather  forecast  is  given  for  24,  sometimes  48  hours 
ahead.  There  has  been  considerable  study  in  trying  to  find  some 
scientific  basis  of  foretelling  the  weather  conditions  some  weeks 
or  months  ahead,  but  no  definite  results  have  been  obtained. 
The  weather  conditions  published  in  certain  pamphlets  and 
almanacs  for  the  entire  year  have  little  if  any  scientific  value. 

322.  Climatic  Zones.— The  surface  of  the  globe  is 
commonly  divided  into  five  climatic  zones,  based  on  an 
arbitrary  division  of  so  many  degrees  of  latitude.  Thus 
the  torrid  zone  includes  all  the  area  between  the  tropics, 
the  two  temperate  zones  the  areas  between  the  tropics  and 
the  polar  circles,  while  the  remainder  is  in  the  frigid  zones. 


394  PHYSICAL    GEOGRAPHY 

It  has  been  shown,  however,  that  the  unequal  distribution 
of  land  and  water  causes  a  distribution  of  winds,  rains, 
and  temperature  that  does  not  follow  the  parallels.  In 
comparing  the  isothermal  chart  of  the  world  for  the  year 
and  for  the  winter  and  summer  seasons  it  will  be  seen  that 
the  temperature  inside  of  the  tropics  in  one  place  is  quite 
different  from  that  in  the  tropics  in  another  place.  A 
comparison  of  the  rainfall  in  different  areas  shows  even 
more  marked  differences.     (See  sec.  293,  fig.  251.) 

A  more  practical  division  of  the  surface  into  tempera- 
ture zones  would  be  based  on  isotherms  rather  than  on 
parallels.  Some  of  the  rather  well  defined  climatic  types 
that  occur  in  different  areas  are  (1)  the  doldrums  of  the 
tropics  with  warm,  moist  climate  and  persistent  rainfall ; 
(2)  the  trade  wind  belt  which  is  warm  and  wet  on  the 
east  side  of  the  continents  and  generally  dry,  sometimes  a 
desert,  on  the  west  side  of  the  continent ;  ( 3 )  the  monsoon 
belt  with  the  wet  and  dry  seasons;  (4)  the  subtropical 
belts  over  which  the  dry  tropical  calms,  the  frequently  pre- 
cipitating trade  winds  and  the  prevailing  westerlies  mi- 
grate at  different  seasons.  The  temperate  zone  may  be 
divided  into  two  parts,  (5)  that  nearer  the  tropics  char- 
acterized by  warm  summers  and  mild  winters,  and  (6) 
the  outer  portions,  by  hot  summers  and  cold  winters. 

There  is  a  marked  difference  between  the  climate  on 
the  seashore  and  that  of  the  interior,  between  the  eastern 
and  western  shores  of  both  the  continents  and  the  oceans. 
Likewise  between  the  plain,  plateau,  and  mountain  cli- 
mates.    Find  some  examples  of  each. 

323.  Changes  in  Climate.— One  frequently  hears  the 
statement  that  the  climate  is  changing— that  there  is  not 
so  much  snow  and  that  the  winters  are  not  so  cold  as  they 
used  to  be.  Such  remarks  apply  to  the  weather  rather 
than  to  the  climate.     There  are  frequently  quite  marked 


THE    ATMOSPHERE  395 

changes  between  successive  seasons,  but  the  official  weather 
records  do  not  indicate  any  marked  changes  in  the  climate 
back  as  far  as  the  record  has  been  kept. 

324.  Geological  Climates.— The  geological  record  which  ex- 
tends over  millions  instead  of  a  few  tens  of  years,  shows  many 
pronounced  climatic  changes.  For  instance,  some  few  thousand 
years  ago  the  climate  was  enough  colder  than  that  at  present  in 
the  northern  hemisphere,  to  cause  an  accumulation  of  snow  and 
ice  in  the  form  of  great  glaciers  over  all  the  north  central  parts 
of  North  America  and  Europe.  This  condition  continued  ap- 
parently for  thousands  of  years.  In  a  preceding  geological  period 
it  was  warm  enough  for  the  growth  of  tropical  plants  as  far 
north  as  the  Arctic  Circle. 

In  still  earlier  geological  times,  a  very  long  time  ago,  the 
climate  of  central  New  York  and  southern  Michigan  was  ex- 
ceedingly dry,  possibly  as  dry  as  that  of  Utah  to-day.  This  is 
shown  in  the  record  by  the  great  beds  of  rock  salt  in  this  region. 

325.  Electric  and  Optical  Phenomena.— Lightning  is 
caused  by  the  electric  discharge  in  the  form  of  a  vivid  flash 
or  spark  between  clouds  or  between  a  cloud  and  the  earth. 
The  lightning  is  associated  with  a  heated  atmosphere  and 
is  common  in  the  hot  summer  season  but  is  absent  in  the 
winter  season,  except  rarely  when  the  air  becomes  unsea- 
sonably warm.  It  appears  also  to  be  associated  with  move- 
ments of  the  warm  air  currents  and  hence  accompanies 
the  violent  air  movements  of  thunderstorms  and  tornadoes. 
Probably  the  moisture  in  the  cloud  is  also  an  important 
element  in  the  electric  discharge.  The  energy,  or  electro- 
motive force,  manifest  in  a  violent  thunderstorm,  is  far  in 
excess  of  that  produced  by  any  artificial  means.  It  takes 
several  different  forms,  known  as  zig-zag  or  chain  light- 
ning, heat  lightning  and  sheet  lightning. 

The  passage  of  the  electric  current  from  the  earth  to  the 
cloud,  or  the  cloud  to  the  earth,  is  likely  to  be  from  some  ele- 
vated point  as  a  tree,  a  church  steeple,  or  some  tall  building, 
yet  this  is  not  always  the  case  as  lightning  has  been  known  to 


396 


PHYSICAL    GEOGRAPHY 


strike  animals   and  other  objects  in  the   near  vicinity  of  trees 
and  buildings  without  injury  to  the  tall  object.     (Fig.  267.) 

The  thunder  is  caused  by  the  inrushing  air  to  fill  the  partial 
vacuum  produced  by  the  lightning  flash.  The  interval  of  time 
between  the  flash  of  the  lightning  and  the  sound  of  the  thunder 
is  an  indication  of  the  distance  of  the  flash. 


Fig.  267.     Lightning  flash,  Lincoln,   Neb.      (U,   G.  Cornell.) 

326.  St.  Elmo's  Fire  is  a  brush  discharge  of  electric- 
ity often  observed  during  electric  storms  on  steeples, 
masts  of  vessels  at  sea,  and  other  high  points.  It  is  un- 
accompanied by  the  noise  or  danger  of  the  lightning  flash. 

327.  The  aurora  horealis  is  presumably  an  electric 
phenomenon,  the  cause  of  which  has  not  been  satisfactorily 
explained.  Since  in  the  northern  hemisphere  it  is  always 
observed  in  the  north  it  is  commonly  called  the  northern 
lights.  It  is  occasionally  observed  as  far  south  as  New 
York,  but  it  is  much  more  frequent  and  spectacular  in  the 
higher  latitudes  where  it  is  an  object  of  much  interest  dur- 


THE    ATMOSPHERE  397 

ing  the  long  northern  winter.  It  consists  of  a  great  arch 
or  sheets  of  light  stretched  across  the  northern  sky,  from 
which  great  streamers  of  many  and  fantastic  forms  ex- 
tend to,  or  towards,  the  zenith.  It  possibly  has  some  rela- 
tion to  the  magnetic  poles  of  the  earth. 

328.  The  rainbow  is  an  arch  of  prismatic  color  that  is 
produced  by  the  refraction  and  reflection  of  the  light  from 
the  interior  of  the  raindrops ;  the  light  emerging  from  the 
drop  is  separated  into  the  prismatic  colors.  Frequently 
a  second  bow  appears  and  even  a  third  and  fourth  have 
been  reported.  A  rainbow  usually  shows  less  than  half  a 
circle.  Under  what  conditions  does  it  appear  a  half  circle  ? 
If  one  could  see  the  rainbow  from  a  balloon  how  much  of 
the  circle  would  appear  ? 

329.  Coronas  or  rings  around  the  moon,  sun  dogs,  moon  dogs, 
and  halos  are  other  phenomena  due  to  refraction  and  reflection 
of  the  light  in  the  upper  atmosphere,  sometimes  from  the  little 
ice  or  snow  crystals,  sometimes  from  drops  of  moisture.  Cor- 
onas are  due  to  diffraction  and  interference.  Halos  are  caused 
by  reflection  and  refraction  of  the  light  in  the  small  ice  crystals. 

The  different  colors  of  the  sky  are  due  to  refraction  and 
selective  scattering  of  the  different  prismatic  rays.  When  there 
is  much  dust  in  the  atmosphere  the  bright  red  and  yellow  colors 
are  reflected  to  the  eye  at  sunrise  and  sunset. 

330.  The  mii-age  is  caused  by  the  turning  of  the  rays 
of  light  from  their  original  direction,  causing  objects  to 
appear  to  be  out  of  place.  It  is  produced  by  the  atmos- 
phere occurring  at  times  in  layers  of  different  density.  The 
light  rays  which  have  already  been  bent  from  their  original 
course  are  reflected  to  the  eye  from  the  surface  of  one  of 
the  layers,  causing  the  object  to  appear  out  of  position  and 
frequently  out  of  proportion. 

The  desert  mirage  occurs  over  hot,  dry  land  areas,  by  the 
reflection  from  the  layers  near  the  earth  to  the  eye,  giving  the 
appearance  of  the  reflection  of  trees  from  a  smooth  water  sur- 
face.    Many  a  person  has  been  lured  to  destruction  by  following 


398  PHYSICAL    GEOGRAPHY 

these  phantom  lakes  across  the  scorching  sands  of  the  desert. 
The  stratification  of  the  lower  layers  of  air  is  due  to  the  intense 
heating  of  the  air  near  the  ground,  which  causes  it  to  expand, 
but  the  air  being  quiet,  there  accumulates  considerable  pressure 
before  convectional  currents  are  started.  Sometimes  the  flight 
of  a  bird  is  sufficient  to  disturb  this  unstable  equilibrium  and 
start  the  uprush  of  heated  air  which  frequently  produces  a  whirl- 
wind.  Sometimes  a  half  dozen  or  more  of  these  whirlwinds  are 
visible  at  one  time  on  the  sandy  plains  of  the  desert  on  a  hot 
dummer  day. 

The  mirage  is  sometimes  visible  on  the  sea,  where  the  reflec- 
tion is  from  the  upper  atmosphere  down  to  the  eye  of  the  ob- 
server, causing  ships  and  other  objects  below  the  horizon  to 
appear  in  the  sky  sometimes  upright  and  sometimes  inverted. 
This  form  of  mirage  is  known  as  the 


331.  The  zodiacal  light  is  a  disk  of  faint  light  sur- 
rounding the  sun.  It  may  be  seen  as  a  triangular  column 
of  light,  rising  from  the  western  horizon  shortly  after  twi- 
light in  the  winter  and  spring  and  in  the  east  before  day- 
break from  September  to  January.  It  is  thought  to  be 
sunlight  reflected  from  a  cloud  of  meteorites  revolving 
around  the  sun.  Another  theory  for  the  zodiacal  light  is 
that  it  is  caused  by  particles  electrically  discharged  from 
the  poles  of  the  sun  and  condensed  along  the  plane  of  its 
equator. 

332.  The  Gegenschein  (counter-glow)  is  a  faint  patch  of  light 
on  the  ecliptic  directly  opposite  the  sun.  One  hypothesis  for  its 
occurrence  is  that  it  is  caused  by  meteors  which  tend  to  con- 
dense directly  opposite  the  sun.  Another  explanation  is  that  it 
forms  a  tail  to  the  earth  similar  to  the  tail  of  a  comet  composed 
of  particles  of  helium  and  hydrogen  escaping  from  the  earth. 

REFERENCES 

1.  Davis,  Elementary  Meteorology.  Ginn  &  Co.,  Boston,  1894. 

2.  Waldo,  Modern  Meteorology,  Scribner's  Sons,  New  York, 

1893. 

3.  Ward,    Practical    Exercises    in    Elementary    Meteorology, 

Ginn  &  Co.,  Boston,  1896. 


THE    ATMOSPHERE  399 

4.  Ferrel,   Popular   Treatise   on  the  Winds,  Wiley    &   SonS; 

New  York,  1889. 

5.  Annual    Reports,    Monthly    Weather    Review    and    Daily 

Weather  Map  by  the  U.  S.  Weather  Bureau,  Wash- 
ington, D.  C. 

6.  Harrington,   Rainfall  and  Snowfall  of  the  United  States, 

Bulletin  C,  U.  S.  Weather  Bureau,  Washington,  D.  C. 

7.  Greely,  American  Weather,  Dodd,  Mead  &  Co.,  New  York, 

1903. 

8.  Ward,  Hann's  Handbook  of  Climatology,  MacMillan  &  Co., 

New  York,  1903. 

9.  Harrington,   Weather  Making,   Ancient   and   Modern,   An. 

Rept.  Smithsonian  Institution,  1894,  pp.  249-271. 

10.  Davis,  Practical  Exercises  in  Geography,  Nat'l  Geog.  Mag. 

Vol.  XI,  p.  62. 

11.  Garriott,  West  Indian  Hurricanes,  Nat'l  Geog.  Mag.,  Vol. 

X,  p.  343,  and  Vol.  XI,  p.  384. 


CHAPTER  XI 
GEOGRAPHY  OF  PLANTS,  ANIMALS,  AND  MAN 

333.  Influence  of  Environment  on  Life.— All  forms 
of  life  are  of  necessity  influenced  by  their  physical  en- 
vironment. The  kind,  the  abundance,  the  variety  of  living 
forms  on  any  area  largely  depends  upon  the  geographical 
conditions  of  soil,  climate,  and  topography  on  the  land ;  and 
temperature,  depth,  and  clearness  of  the  waters  in  the 
sea.  This  has  been  suggested  from  time  to  time  in  the  pre- 
ceding chapters,  but  it  seems  fitting  now  in  conclusion  to 
consider  the  subject  directly  in  reference  to  the  life  rela- 
tions. 

Man  is  probably  as  nearly  independent  of  his  geo- 
graphical surroundings  as  any  other  form  of  life,  but  that 
in  his  migrations,  civilization,  industries,  and  mental  as 
well  as  physical  development,  he  has  been  greatly  influ- 
enced by  geographical  conditions  is  apparent  to  all.  Many 
of  the  lower  forms  of  life  are  more  susceptible  than  man 
to  their  surroundings  and  hence  occupy  only  a  few  limited 
areas,  while  man  ranges  over  the  earth  from  the  equator 
nearly  to  the  pole  and  is  making  strenuous  efl'orts  to  reach 
that  hitherto  inaccessible  point. 

334.  Effect  of  Climate.— There  are  striking  differ- 
ehces  in  the  kinds  of  life  and  the  habits  of  the  living  forms 
in  the  different  climatic  zones.  In  the  warm,  humid 
region,  life,  death,  and  decay  go  on  with  striking  uniform- 
ity and  rapidity  throughout  the  year  and  the  years.  In 
the  cold  temperate  zones  there  is  a  warm  season  of  rapid 
growth,  and  a  cold  season  of  rest,  when  the  trees  and  shrubs 

400 


n 


GEOGRAPHY    OF    LIFE 


401 


shed  their  leaves  and  fruit,  and  the  herbs  and  grasses  die 
and  disappear  all  but  the  roots,  bulbs,  and  seeds.  Many 
of  the  animals  hibernate.  Man,  the  domestic  animals,  and 
some  of  the  wild  animals  remain  active  during  the  cold 
weather  of  the  winter  months,  but  the  lower  forms  of 
life,— many  of  the  animals,  and  all  the  vegetable  forms— 
lie  dormant  and  inactive  until  the  return  of  warm  weather. 
The  winter  in  cold  climates  is  characteristically  a  sea- 
son of  silence.  At  a  distance  from  human  habitations  al- 
most the  only  sounds  are  those  of  inanimate  nature. 

"With  the  coming  ot  the  spring  there  is  a  marvellous  awak- 
ening and  unfolding.    The  brooks,  swollen  to  overflowing  by  the 


Fig.   268.     Winter  in  cold  climates  is  a  season  Oi  silence.     Winter  scene  in  an 
evergreen  forest  in  the  United  States. 


melting  of  the  snow,  make  music  as  they  run.     The  northward 
flight  of  the  birds  brings  to  every  grove  a  chorus  of  song.     A 
host  of  batrachians  and  reptiles  bestir  themselves  after  a  long 
26 


402  PHYSICAL    GEOGRAPHY 

winter  sleep  and  vociferously  proclaim  their  presence.  The  in- 
sect world,  with  its  unnumbered  legions,  takes  wing.  The  air 
vibrates  with  millions  of  voices.  The  trees  put  forth  their  leaves, 
each  a  harp-string  which  responds  to  the  touch  of  the  fingers  of 
the  wind.  The  organ-notes  of  the  thunder  again  startle  the  hiber- 
nating echoes.  As  the  winter  is  the  silent  season,  so  the  spring 
is  the  time  of  music."     (Russell's  North  America,  pp.  296-297.) 

Make  a  list  of  the  birds  and  wild  animals  you  see  or 
know  to  be  alive  in  our  fields  and  forests  in  the  winter. 
Note  the  date  when  you  see  the  first  birds  in  the  spring 
and  the  ones  that  come  first.  What  animals  hibernate  or 
sleep  during  the  cold  season?     (See  fig.  268.) 

This  seasonal  renewal  of  the  activities  of  the  varied 
forms  of  life  is  probably  one  of  the  reasons  why  man  has 
made  his  greatest  advancement  in  the  temperate  zones. 

PLANT  GEOGRAPHY 

The  number  and  kinds  of  plants  on  any  area  not  under 
cultivation  are  determined  largely  by  the  condition  of  the 
soil,  w^ater,  air,  and  temperature. 

335.  Soil. — Most  land  plants  have  roots  which  find 
anchorage  in  the  soil  from  which  they  derive  sustenance 
both  in  water  and  mineral  matter.  While  but  a  small  part 
of  the  plant  is  formed  by  the  mineral  matter  in  the  soil, 
that  small  part  is  so  important  that  if  the  materials  are 
not  in  the  soil  the  vegetation  does  not  flourish.  Thus  a 
grain  of  wheat  that  would  sprout  and  grow  a  spindly  stalk 
a  foot  high  with  no  grains  on  poor  soil,  would  grow  a  lusty 
stalk  four  feet  high,  with  many  good  grains,  on  a  fertile 
soil. 

The  kind  of  soil  has  much  to  do  with  the  variety  and  quan- 
tity of  vegetation.  Thus  the  vegetation  on  a  sand  soil  will  be 
different  from  that  on  a  rock,  clay,  loam,  humus,  or  alkaline  soil. 
Much  depends  on  the  relation  of  these  soils  to  each  other;  thus 
a  humus  on  sand  would  be  different  from  a  humus  on  clay.   More 


GEOGRAPHY    OF    LIFE 


403 


Important  than  the  chemical  proportions  are  the  physical  ones, 
such  as,  the  fineness  of  the  particles  and  the  porosity  of  the  mass 
as  affecting  the  circulation,  absorption,  and  retention  of  moisture. 
Some  forms  of  life  are  independent  of  the  soil,  such  as  floating 
vegetation,  which  derives  sustenance,  wholly  from  the  water  and 
air.    A  few  land  plants  live  without  contact  with  the  soil  and 


jjiG,  269.     Spanish   moss  on   live  oak   tree,    Columbia, 
A  common  epiphyte  in  the  southern  United  States. 


(W.  L.  Bray.) 


derive  sustenance  wholly  from  the  air.  Such  are  called  epiphytes, 
because  they  grow  upon  the  stems  and  branches  of  other  plants. 
They  are  most  abundant  in  the  tropics.  Fig,  269  shows  the 
Spanish  moss  an  epiphyte  that  grows  in  great  abundance  in  the 
southern  and  southwestern  United  States.  The  mistletoe  is  a 
common  epiphyte  widely  distributed  through  the  United  States 
and  Europe. 

336.     Water.— Water  forms  a  large  part  of  the  material 

of  nearly  all  plants  and  hence  is  essential  to  their  existence. 
The  kind  of  water,  whether  fresh,  salt,  or  alkaline,  and  the 
quantity  of  it,  whether  an  excess  as  in  the  marsh,  a  dearth 


404 


PHYSICAL    GEOGKAPHY 


as  in  the  desert,  a  limited  supply  as  in  the  semi-arid  dis- 
tricts,  or  a  generous  amount  as  in  the  humid  districts ;  and 
the  temperature,  whether  hot,  temperate,  or  cold,  determine 
in  a  large  degree  the  kind  as  well  as  the  quantity  of  the 
vegetation.  The  distribution  of  the  rainfall  throughout 
the  year  is  important;  whether  it  falls  in  the  winter  or  in 
the  growing  season.  The  relation  of  the  water  table  to 
the  surface  soil  is  also  an  important  factor. 


Fig.  270.  Some  types  of  water  plants  growing  in  a  swamp  of  northern 
United  States.  Some  float  on  the  surface,  others  extend  several  feet  above 
the  surface,  while  others  grow  in  the  bottom  entirely  under  water.  View 
in  the  Montezuma  Swamp,   N.  Y.      (E.  R.   Smith.) 

337.  Classification  of  Plants  Based  on  Water  Supply. 
— On  a  basis  of  humidity  plants  may  be  divided  into  (1) 
water  plants,  (Hydrophytes)  ;  (2)  drouth  or  desert  plants 
(Xerophytes)  ;    (3)   intermediates   (Mesophytes). 

1.     Some  water  plants  grow  on  the  surface  of  the  water, 


GEOGRAPHY    OF    LIFE 


405 


others  on  the  bottom.     Another  class  includes  those  with 
roots  on  the  bottom  and  leaves  and  branches  above  the  sur- 


FlG.  271.  Illustrating  the  function  of  cypress  knees.  A,  level  of  water  in 
the  growing  season.  B,  lowest  level  of  swamp  water;  a,  cypress  tree  with 
part  of  the  roots  under  water;  b,  tree  with  all  roots  under  water;  c,  tree 
with  none  of  the  roots  under  water;  dd,  cypress  knees  through  which  the 
roots  breathe;  ee,  knees  not  yet  gi'own  to  serviceable  height;  ff,  abortive 
knees  not  needed  by  the  tree.      (After  Shaler.) 


Fig.  272.  Cypress  trees  and  knees  in  the  Great  Dismal  Swamp  of  Virginia. 
The  knees  are  protuberances  on  the  roots  extending  sometimes  to  several 
feet  above  the  surface  of  the  water.     (U.  S.  Geol.  Survey.) 

face,  such  as  the  mangrove  tree  in  Florida,  water  lilies,  cat- 
tails, splatter  dock,  reeds,  and  cane  in  the  lakes  and 
swamps.     The  water  hyacinth  is  a  floating  plant  that  grows 


OF  THE 

UNIVfiRSITY 


406 


PHYSICAL   GEOGRAPHY 


Via.  273.  Papago  Indian,  Sonora,  Mexico,  crushing  the  pulp  of  the  interior 
of  a  barrel  cactus  in  order  to  squeeze  the  water  out  of  it.  Sometimes  a 
gallon  or  more  of  water  is  obtained  in  this  way  from  a  single  plant. 
(D.  T.  McDougal.) 


GEOGRAPHY    OF    LIFE  407 

in  such  quantities  on  the  rivers  and  lakes  of  Florida  as  to 
be  a  serious  menace  to  navigation  and  the  fishing  industries 
on  the  inland  waters.      (Fig.  270.) 

Cypress  trees  that  grow  sometimes  in  the  water  and  some- 
times on  dry  land  develop  a  unique  method  of  getting  air  to  the 
roots  that  grow  under  water,  by  having  a  knob  or  process  grow 
up  through  the  water  to  and  above  the  surface  until  it  is  in  con- 
tact with  the  air.  These  knobs  which  are  known  as  "knees," 
stand  above  the  surface,  looking  much  like  stumps.  (See  figs. 
271  and  272.)  After  the  drainage  of  the  swamp  the  new  growth 
of  cypress  on  the  dry  land  is  devoid  of  knees. 

The  Sargassum  that  occurs  so  abundantly  out  in  mid-ocean 
has  numerous  small  air  sacs  which  serve  as  floats  or  life-pre- 
servers to  keep  it  at  the  surface.  The  Bladderroot,  a  fresh  water 
plant,  has  similar  floats.  The  bladders  or  air  sacs  serve  to 
aerate  the  plant  as  well  as  float  it.  The  duckweed,  a  small  green 
floating  plant  has  numerous  air  chambers  through  the  body  of 
the  plant.  There  is  another  class  of  water  plants,  microscopic 
in  size,  that  occurs  in  vast  quantities  in  both  salt  and  fresh  water. 
The  best  known  forms  in  this  class  are  the  diatoms  which 
secrete  a  wall  of  silica  and  hence  are  preserved  in  great  deposits 
in  the  bottom  of  the  sea  and  lakes.     (See  sec.  105.) 

2.  Desert  plants  which  have  the  opposite  condition 
from  the  water  plants,  have  many  ways  for  collecting,  re- 
taining and  conserving  the  moisture  and  prolonging  their 
existence  in  the  absence  of  rain.  The  scarcity  of  leaves 
is  one  device,  as  it  is  from  the  leaves  that  evaporation 
takes  place.  Some  of  the  cacti  have  no  leaves.  A  hairy 
covering  over  the  leaves  in  some  plants  serves  to  shade  thera 
from  the  rays  of  the  sun.  Thorns  and  brambles  serve  to 
protect  many  of  them  from  being  eaten  by  animals.  The 
cactus,  sagebrush,  greasewood  and  yucca  are  among  the 
most  common  plants  on  our  western  deserts.  The  semi- 
arid  regions,  which  are  subject  to  drought  at  regular  or  ir- 
regular periods,  have  a  greater  variety  and  number  of 
plants  than  the  desert.  In  such  areas  some  of  the  plants 
store   water    in   reservoirs   provided   for   that   purpose   in 


408 


PHYSICAL    GEOGRAPHY 


GEOGRAPHY    OF    LIFE 


409 


the  leaves  and  stems.  Others  conserve  moisture  by  curling 
the  leaves  and  thus  exposing  a  smaller  surface.  (Figs. 
273,  274,  and  275.) 


Fig.  275.  Oasis  of  palms  {Neo  Washingtonia  filifera)  in  the  Colorado  Desert, 
near  Indio,  Cal.  (See  FiG.  79.)  Any  part  of  a  desert  area  with  water  at 
or  near  the  surface  is  an  Oasis.  It  is  a  green  spot  in  the  midst  of  the 
brown  waste.      (D.  T.  McDougal.) 

3.  The  intermediate  plants  that  grow  on  dry  land  sup- 
ported by  a  fairly  abundant  rainfall  are  the  most  numerous 
and  comprise  about  80  per  cent  of  the  total  flora.  They 
are  called  mesophytes  because  they  occur  midway  between 
the  very  dry  and  the  very  wet  conditions.  They  include 
most  of  our  cultivated  plants  and  most  of  the  forest  trees. 
They  occur  in  the  same  rain  belts  as  the  water  plants  but 
under  different  physiographic  conditions,  that  is,  where 
there  is  sufficient  rainfall  but  the  water  does  not  stand  on 
the  surface  as  in  the  case  of  the  hydrophytes. 

They  occur  in  different  rain  belts  from  the  desert  plants. 
The   dividing  line  is  about  15  inches  annual  rainfall.     It  varies 


410 


PHYSICAL    GEOGRAPHY 


considerably  with  the  distribution  of  the  rainfall  and  other  con- 
ditions. It  requires  about  20  inches  of  annual  rainfall  to  support 
forest  growth,  but  under  certain  conditions  some  coniferous  trees 
exist  in  cold  climates  on  less  than  that. 

The  slopes  of  the  Rocky  Mountains  are  covered  with  forests 
(where  they  have  not  been  destroyed)  which  form  a  belt  between 


Fig.  276.  View  at  the  timber  line  in  the  Rocky  Mountains,  Ouray  County, 
Colo.  The  upper  portion  of  the  mountains  are  void  of  trees.  The  forests 
are  dense  towards  the  base  of  the  mountain.  Snow  lies  on  the  high  moun- 
tains nearly  all  the  year. 


the  plains  at  the  base,  treeless  from  lack  of  rainfall,  and  the  tree- 
less peaks  at  the  top  made  so  from  excess  of  snow.  On  all  very 
high  mountains  even  in  the  tropics  there  is  an  upper  limit  to 
tree  growth  known  as  the  timber  line.     (See  fig.  276.) 

The  upper  limit  of  trees  is  not  due  directly  to  cold  but  to 
excess  of  snow,  as  shown  by  the  occurrence  of  trees  on  the  nar- 
row ridges  much  higher  than  in  the  depressions.  It  is  in  the 
depressions  where  the  snow  accumulates  and  remains  long  after 
it  has  melted  from  the  ridges,  so  that  the  growing  season  when 
the  ground  is  free  from  snow  is  too  short  for  trees  to  develop. 

In  respect  to  their  association,  there  are  two  great  groups  of 
the  intermediate  plants  that  have  a  very  pronounced  effect  on 


GEOGRAPHY    OF    LIFE 


411 


the  surface.  The  one  group  consists  of  grasses  and  Jierhs  and 
forms  the  meadows,  prairies,  pastures,  and  tundras.  The  other 
consists  of  shrubs  and  trees  and  forms  the  thickets  and  forests. 
With  change  of  conditions  each  of  these  may  encroach  upon  the 
territory  of  the  other.  Fig.  277  shows  an  area  in  Texas  where 
the  forest  trees  are  now  advancing  over  the  grass  plains.  The 
axe  and  forest  fires  have  been  instrumental  in  changing  thousands 
of  acres  of  forest  area  to  grass  land,  or  in  some  cases  to  waste 
land. 


Fig.  277.  Forest  growth  advancing  on  the  praine,  Tarkmgton  Prairie,  Liberty 
County,  Texas.  (W.  L.  Bray.)  Where  the  prairies  are  bordered  by  forests, 
the  tendency  is  in  some  places  for  the  forests  to  advance  on  the  prairie. 


338.  Air  and  Light.— The  chief  supply  of  raw  ma- 
terials for  the  plants  is  derived  from  the  air  and  consists 
largely  of  carbonic  acid,  which  in  the  cells  of  the  green 
plant  is  decomposed,  the  carbon  with  some  oxygen  and 
hydrogen  forming  compounds  which  make  up  the  plant 
tissue,  while  some  of  the  oxygen  is  set  free.  The  nitrogen 
of  the  plant  comes  from  the  soil  but  the  soil  probably  ob- 
tained it  originally  from  the  air. 


Fig.    278.      Sequoi.  (Jeol.    Survey.)      The    big   trees,    small 

trees,   and  shruDs  {jrow    in   (nm-rein    light  zones.      The  big  trees  shade  the 
smaller  ones,    and  both    shade  the  shrubs    and   ground   plants. 


GEOGRAPHY    OF    LIFE  413 

All  green  plants  require  light.  Green  cells  are  food 
factories  where  water,  materials  from  the  soil,  carbon 
dioxide  and  other  gases  of  the  air  are  combined  to  produce 
food  for  animals  or  material  for  other  plants.  The  sun- 
light and  the  green  material  (called  chlorophyll)  of  the 
plant  cell  seem  to  be  the  most  important  factors  in  this  lit- 
tle organic  laboratory  where  the  inorganic  air  and  mineral 
matter  are  changed  to  the  organic  vegetable  compounds. 
The  plant  tissue  which  is  eaten  by  animals  undergoes 
further  changes  in  the  chemical  laboratory  of  the  animal 
which  devours  it,  and  it  is  there  transformed  to  animal 
tissue.  Some  plants  known  as  light  plants  require  more 
light  than  others  which  are  known  as  shade  plants.  In  a 
forest  there  are  several  zones  or  strata  based  on  the  rela- 
tive amount  of  light.  The  tall  trees  form  the  upper  zone 
and  receive  the  most  light,  below  this  is  a  stratum  of 
shrubs,  then  herbs,  and  next  to  the  ground  the  green  mosses 
and  lichens.     (See  fig.  278.) 

339.  Temperature.— The  extremes  of  temperature  be- 
tween which  nearly  all  plants  grow  are  32  and  122  degrees 
F.  Some  forms  of  algae  live  in  the  Hot  Springs  of  Yel- 
lowstone Park  at  a  temperature  as  high  as  199  degrees. 
By  a  special  adaptation  to  change  of  conditions,  plants 
lying  dormant  pass  through  cold  winter  seasons  having  a 
temperature  much  below  32  degrees.  The  distribution  of 
the  temperature  throughout  the  year,  and  the  relation  of 
the  temperature  to  moisture  are  important  factors  to  con- 
sider. 

A  general  subdivision  of  the  land  area  into  plant  zones  based 
on  temperature  is  as  follows: 

1.  Boreal,  polar  or  cold  zone,  with  mean  annual  temperature 
below  30  degrees  F.,  contains  lichens,  mosses,  gentians,  willows, 
etc.  It  includes  the  greater  part  of  North  America  north  of  the 
United  States  and  high  mountain  areas  in  the  United  States. 


414 


PHYSICAL    GEOGRAPHY 


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GEOGRAPHY    OF    LIFE  415 

2.  Transition,  or  cold  temperate,  mean  annual  temperature 
30  to  40  degrees  F.,  contains  evergreens,  such  as  spruce,  fir,  hem- 
lock, pine,  etc.  This  zone  forms  a  fairly  wide  belt  along  the 
northern  United  States,  southern  Canada,  and  along  the  Alle- 
ghany, Rocky,  and  Sierra  Nevada  mountain  ranges,  between  the 
boreal  on  the  mountain  top  and  the  austral  of  the  bordering 
plains. 

3.  Upper  Austral,  or  warm  temperate,  mean  annual  tempera- 
ture 40  to  60  degrees  F.,  contains  'deciduous  trees  such  as  oak, 
maple,  beech,  chestnut,  etc.  It  covers  the  central  United  States, 
including  the  greater  part  of  the  plains  and  prairies  and  portions 
of  the  Alleghany  plateau.  It  may  be  divided  into  the  arid  plains 
region  of  the  west  and  the  humid  areas  of  the  middle  and  east. 

4.  Lower  Austral,  or  sub-tropical,  mean  annual  temperature 
60  to  72  degrees  F.,  contains  broad  leaved  evergreens,  magnolia, 
holly,  cactus,  pines,  palmettos,  and  cypress.  It  includes  the  At- 
lantic coastal  plain  south  of  the  Potomac,  the  Gulf  plains,  lower 
Mississippi  Valley,  Texas  east  of  the  staked  plains,  part  of 
Arizona,  and  the  low  areas  of  southern  and  central  California. 

5.  Tropical,  mean  annual  temperature  72  to  82  degrees  F., 
has  a  luxuriant  vegetation,  containing  great  numbers  of  climb- 
ing and  air  plants.     It  occurs  in  southern  Florida  and  Cuba. 

Local  Zonation.  Besides  the  broad  planetary  zones  of  tem- 
perature just  described,  there  are  many  local  zones  in  each,  some 
based  on  temperature,  some  on  moisture,  some  on  dependence 
upon  other  plants,  some  on  light,  and  others  on  the  kind  of  soil 
or  rock. 

Most  all  large  hills,  and  mountains  are  belted  with  different 
kinds  of  plants  or  trees,  the  belts  being  very  irregular  in  many 
places,  as  the  zones  are  determined  in  part  by  the  temperature 
due  to  elevation,  in  part  by  light  and  wind,  in  part  by  the  kind 
of  soil. 

Most  all  swamps  and  some  lakes  have  concentric  zones  based 
upon  the  depth  of  water.     (See  figs.  85,  90  and  92.) 

340.  Control  of  Plant  Distribution  by  Methods  of 
Migration. — On  areas  in  which  the  temperature  and 
humidity  favor  vegetation,  the  plants  must  be  distributed 
over  the  area  in  some  way.  In  the  first  uplift  of  a  lake 
bed,  a  coastal  plain,  or  an  island  area  there  may  be  no  veg- 


416  PHYSICAL    GEOGRAPHY 

elation.     Seeds  of  land  plants  may  spread  over  it  in  va- 
rious ways  as  follows: 

1.  Some  are  carried  by  the  wind.  Some  seeds  like 
those  of  the  thistle  and  dandelion  have  feathery  floats 
which  serve  as  little  balloons  to  buoy  up  the  seed  so  that  it 
may  be  carried  long  distances  before  it  falls  to  the  earth 
to  sprout  and  start  a  new  center  of  distribution. 

2.  Some  seeds  have  spines  or  sharp  prongs  by  which 
they  are  attached  to  the  fur  or  hair  of  animals  and  thus 
carried  to  distant  points.  Such  are  the  burdocks,  and 
Spanish  needles. 

3.  Edible  seeds  or  seeds  in  edible  fruit  are  often  car- 
ried by  birds  or  other  animals  to  distant  points,  and  there 
grow  and  multiply. 

4.  Some  plants  have  explosive  seed  pods  that  fly  open 
with  force  and  throw  the  seeds  some  distance  away.  A 
repetition  of  this  process  from  year  to  year  carries  the 
plants  over  wide  areas.     Study  the  common  witch  hazel. 

5.  Seeds  and  sometimes  plants  are  carried  long  dis^ 
tances  by  rivers  and  ocean  currents.  Many  oceanic  islands 
obtain  their  plants  in  this  way.  The  seeds  of  the  cocoa 
palm  are  thought  by  some  to  be  widely  distributed  by  the 
ocean  currents  among  the  coral  islands  of  the  tropical  Pa- 
cific Ocean. 

6.  Man  is  one  of  the  most  important  agents  in  dis- 
tributing plants.  He  transplants  them  to  distant  parts  of 
the  world,  over  mountains  and  across  oceans  or  deserts. 
The  food  and  flowering  plants  are  carried  to  all  lands  for 
cultivation,  and  the  seeds  of  weeds  and  other  undesirable 
plants  are  unavoidably  carried  with  them.  The  railroads 
and  automobiles  are  important  agents  in  this  distribution. 
Many  plants  migrate  or  spread  by  sending  out  shoots,  run- 
ners or  underground  stems.  The  strawberry  is  a  good 
example. 


GEOGRAPHY    OF    LIFE 


417 


Certain  plants  at  times  show  a  very  peculiar  distribution. 
The  Hart's-tongue  fern  (Fig.  280)  occurs  in  Onondaga  county, 
N.  Y.,  in  a  few  places  only  on  the  Onondaga  limestone.  It  is  not 
known  to  grow  on  any  other  rock  in  the  county;  nor  is  it  known 
to  occur  in  any  other  place  in  eastern  United  States  except  one 
place  in  Tennessee.  It  occurs  in  Ontario  and  is  common  in 
North  Africa  and  Southern  Europe. 

341.     Barriers.— It  seems  probable  that  the  different 


Fig.  280.     Hart's   Tongue  fern    {Scolopendrium   officinarum)    occurs  in   Onon- 
daga County,   N.  Y.,   only  on   Onondaga  limestone.      Tennessee   is  the  only 
other  locality  where  it  is   known   to   occur  in  the  United   States.      (J.   E. 
Kirkwood.) 
27 


418  PHYSICAL    GEOGRAPHY 

species  of  plants  and  animals  had  each  a  starting  point  on 
one  of  the  continents  from  which  it  spread  over  the  sur- 
rounding region  until  it  met  some  barrier  which  checked 
or  stopped  its  further  advance.  Owing  to  the  varied  habits 
of  different  species,  what  would  prove  a  barrier  to  one 
form  might  be  no  barrier  to  another. 

The  question  of  barriers  which  check  the  spread  of 
animals  or  plants  from  the  center  of  origin  is  one  of  great 
interest  but  involved  in  too  many  complexities  for  thorough 
discussion  here. 

Animals  or  plants  that  live  on  the  bottom  of  the  shallow 
portions  of  the  sea  on  the  continental  shelf,  find  a  barrier 
in  the  land  on  one  side  and  the  deep  ocean  on  the  other. 
Between  these  two  barriers  the  species  range  along  the 
shore  until  stopped  by  an  obstruction  of  some  other  kind, 
the  most  common  one  being  a  change  in  temperature.  Some 
forms  can  live  only  in  cold  water,  others  only  in  warm 
water,  and  others  only  in  temperate  water.  Thus  the  reef- 
building  coral  polyp  which  cannot  stan'^  a  temperature  be- 
low 68  degrees  F.  is  limited  by  the  temperature  to  tropical 
regions. 

To  most  of  the  plants  and  animals  that  live  in  the  sea, 
fresh  water  is  a  barrier,  and  they  do  not  enter  the  river 
however  deep  and  wide  the  channel  may  be.  The  opposite 
is  true  for  many  fresh-water  forms ;  that  is,  the  salt  water 
at  the  mouth  of  a  river  is  an  effective  barrier  that  prevents 
the  spread  into  other  rivers.  There  are  a  few  forms,  how- 
ever, that  live  in  either  fresh  or  salt  water.  To  such  forms 
the  shore  line  is  not  a  barrier,  and  they  may  pass  along  the 
shore  from  river  to  river  until  they  reach  a  temperature 
barrier. 

The  shore  line  of  a  large  body  of  water  is  a  barrier  to 
most  animals  and  plants  that  live  on  the  land  as  well 
as  to  those  that  live  in  the  water.     Many  of  the  land  ani- 


GEOGRAPHY    OF    LIFE  419 

mals  can  cross  a  river  or  small  lake  by  swimming  but  they 
find  the  ocean  an  effective  barrier. 

Mountains,  especially  high  mountain  ranges,  are  effective  bar- 
riers to  most  of  the  forms  of  land  life  both  plant  and  animal. 
For  that  reason  the  indigenous  life  on  the  two  sides  of  such  a 
mountain  range  as  the  Rockies  or  the  Alps  is  likely  to  be  quite 
different.  To  most  forms  of  plants  of  the  austral  and  transition 
zones  the  boreal  area  of  a  high  mountain  range  is  an  insurmount- 
able barrier.  Seeds  with  balloon  attachments  like  the  thistle 
may  sometimes  be  carried  over  by  the  wind,  and  burs  and  other 
seeds  with  hooks  may  be  carried  across  in  the  fur  of  animals, 
but  most  of  the  plants  at  the  base  of  the  mountains  have  no 
natural  means  ot  getting  over  the  crest. 

Deserts,  especially  large  deserts  such  as  the  Sahara  or  those 
of  Central  Asia,  are  effective  barriers  to  most  forms  of  life.  The 
aridity  of  the  desert  is  as  destructive  to  life  as  the  cold  tem- 
perature of  the  mountain  tops.  Man,  by  provident  forethought, 
may  carry  sufficient  supplies  of  food  and  water  to  enable  him  to 
cross  to  supplies  on  the  other  side,  but  not  so  the  plant  or  the 
wayward  animal,  which  perishes  for  want  of  water. 

Plains  are  barriers  to  certain  forms  of  life  that  flourish  only 
on  the  mountains.  The  broad  stretches  of  prairies,  the  treeless 
plains  of  the  Mississippi  Valley,  prove  a  more  effective  barrier 
than  the  great  rivers  to  denizens  of  the  forests  and  the  hills. 

Life  of  one  kind  may  prove  a  very  effective  barrier  to  other 
forms  of  life. 

A  forest  is  a  barrier  to  certain  kinds  of  plants  and  animals, 
while  its  shade  is  necessary  for  the  life  of  others.  So  a  thicket 
is  a  barrier  to  some  forms  and  a  protection  to  others.  To  some 
the  meadow  or  the  grassy  prairie  is  a  decided  check. 

Some  forms  of  life  are  dependent  on  others  and  cannot 
flourish  without  them.  To  such  dependent  forms  the  absence  of 
the  godfather  on  which  they  depend  is  a  serious  negative  barrier 
to  their  advance. 

Certain  insects  are  necessary  for  the  fertilization  of  certain 
plants,  and  destruction  of  either  the  plants  or  the  insects  would 
cause  a  destruction  of  the  other. 

Some  forms  are  enemies  to  others  and  where  they  occupy 
an  area  they  prove  a  decided  barrier  to  the  advance  or  spread 
of  the  new  form  in  that  direction. 


420  PHYSICAL    GEOGRAPHY 

342.  Effect  of  Vegetation  on  Physiography.—  Some 
of  the  effects  of  vegetation  on  shore  lines  have  been  described 
in  Chapter  VI.  The  growth  of  the  mangrove,  the  eel  grass, 
marsh  grass  and  other  water  plants  frequently  produces 
very  marked  changes  on  the  position  of  the  shore  line  and 
in  the  case  of  small  lakes,  their  results  are  not  limited  to 
the  shore  but  they  fill  the  entire  lake  or  marsh  and  make 
a  fertile,  dry  land  area  of  it. 

The  drifting  vegetation  frequently  lodges  and  forms  an 
obstruction  on  the  course  of  a  stream  where  later,  sand  and 
gravel  are  deposited,  sometimes  turning  the  river  from  its 
channel. 

343.  Indigenous  and  Existent  Vegetation.—  One 
should  keep  in  mind  constantly  the  distinction  between  ex- 
istent vegetation  and  the  indigenous.  The  latter  refers  to 
the  plants  which  are  native  to  the  area  through  geographic 
influences  independent  of  man.  The  existent  vegetation 
is  often  in  large  measure  the  result  of  man's  influence. 

The  potato,  the  maize  or  Indian  corn,  and  tobacco  are 
indigenous  to  America  but  are  now  widely  distributed  over 
the  world.  The  peach,  fig,  grape,  and  orange  are  indigen- 
ous to  the  European  continent  but  are  now  distributed  in 
all  countries. 

Not  only  are  fruits,  grains  and  vegetables  widely  dis- 
tributed by  man,  but  many  weeds  and  flowers  as  well.  The 
thistle,  for  instance,  has  become  so  abundant  in  many  local- 
ities as  to  cause  laws  to  be  made  prohibiting  any  one  from 
permitting  it  to  grow  on  his  land. 

The  beautiful  scarlet  geranium,  cultivated  in  our  gar- 
dens and  greenhouses,  is  indigenous  to  South  Africa,  but  it 
has  been  transplanted  by  man  to  all  parts  of  the  world. 
It  is  now  growing  wild  in  great  luxuriance  in  California, 
Australia  and  elsewhere. 

The  following  is  a  list  of  plants  with  the  name  of  the  country 


GEOGRAPHY    OF    LIFE  421 

in  which  they  are  indigenous.  From  this  make  a  list  of  such  as 
you  know  to  be  growing  where  you  live,  stating  in  each  case  the 
way  or  ways  in  which  you  think,  it  may  have  been  transplanted 
from  its  original  habitat. 

Fruits,  etc.  Apple — Europe.  Apricot — America  or  China. 
Banana — S.  Asia.  Black  Currant — Europe.  Cherry — Asia  Minor. 
Gooseberry — England.  Mango — S.  Asia,  Malay  Peninsula.  Oranges 
and  Lemons — Cochin  China,  Indo-China.  Pear — Australia,  all  over 
Europe.  Plum — common,  Mt.  Elbons,  N.  Persia.  Pineapple — 
Brazil,  Mexico,  Guiana.  Pomegranate — Persia,  Afghanistan,  Bel- 
uchistan.  Quince — N.  Persia.  Raspberry — Temperate  Europe 
and  Asia.  Red  Currant — England  and  Normandy.  Strawberry — 
Europe,  Asia,  N.  America.  Tomato — Peru.  Half  a  century  or 
more  ago  they  v/ere  considered  poisonous  and  raised  for  orna- 
ment. Used  to  frighten  the  slaves  before  the  war.  Fig — Mediter- 
ranean Basin.  Grape — Cultivated  first  probably  in  Asia,  wild  in 
N.  America,  Europe  and  Asia.  Date  Palm — Narrow  zone  from 
Euphrates  to  Canaries.  Peanut — Brazil.  Cloves — Moluccas.  Red 
Pepper — America. 

Vi7ie  fruits.  Cucumber — ^West  Indies.  Gourd — Coast  of  Mal- 
abar and  Abyssinia.  Musk  Melon — Asia,  Mexico,  or  California. 
Pumpkin — Mexico   or  Texas.     Water  Melon — Egypt. 

Nuts — fruits.  Cocoa-nut  palm — America.  Chestnut — Temper- 
ate America,  Europe,  Japan.  Hickory — 10  species  all  in  E.  North 
America,  Canada,  Mexico,  Walnut — North  America,  temperate 
Asia,  S.  E.  Europe.  Occurs  fossil  in  Tertiary  and  Quaternary  in 
North  America. 

Miscellaneous  fruits.  Cactus,  Prickly  Pear — Native  in  Mexico, 
used  for  fruit  and  fodder,  spineless  variety  now  being  cultivated 
in  arid  areas  in  United  States.  Hops — Eurasia.  Lima  Bean — 
Peru.  Introduced  in  United  States  about  1820.  Poppy — Shores 
of  Mediterranean. 

Roots  and  tubers.  Jerusalem  Artichoke — Canada  and  United 
States.  Beet — Canary  Islands,  Mediteranean  Basin.  Carrot 
— Europe  and  West  Temperate  Asia.  Potato — United  States 
of  Columbia  and  Peru.  Radish — W.  Temperate  Asia.  Sal- 
sify— Borders  of  Mediterranean.  Sweet  Potato — America,  some 
say  Asia.     Turnip — Europe,  Siberia. 

Grains  and  seeds.  Barley — Eurasia.  Buckwheat — Manchuria, 
near  Lake  Baikal.     Maize  (Indian  corn) — America.     Oats — Europe 


422  PHYSICAL    GEOGRAPHY 

and  Tartary.  Rice — India  or  China.  It  was  used  in  China  2,800 
B.  C.  Rye — Between  Austrian  Alps  and  Caspian  Sea.  Wheat — 
Uncertain,  very  ancient,  probably  Euphrates  Valley.  Antedates 
historic  records.  Known  in  China  2,700  B.  C.  Found  in  Pyra- 
mids of  Egypt  of  date  thought  to  be  2,700  B.  C.  Durum  Wheat 
— ^Long  used  in  Russia  and  Southern  Europe.  Introduced  in 
U.  S.  and  Canada  about  1900.  Much  more  productive  on  the  dry 
lands  of  the  wheat  belt  than  common  wheat.     Coffee — Abyssinia. 

Fibers.  Cotton — South  Asia.  Flax — Borders  Mediterranean, 
Caspian  and  Black  seas.     Hemp — Central  Asia,  Siberia. 

Plants  used  for  stems  or  leaves.  Artichoke  (true) — Borders  of 
Mediterranean.  Asparagus — Europe,  England,  W.  Temperate 
Asia.  Alfalfa — Brought  to  the  United  States  from  Chile.  Prob- 
ably from  Asia  Minor  or  Arabia.  Celery — Europe  and  Asia. 
Cabbage — Europe.  Clover,  Purple — Asia,  Aryan  Nations;  crim- 
son— around  Pyrenees.  Lettuce — S.  Europe,  Canary  Islands, 
Algeria,  E.  Asia.  Millet — Egypt,  Arabia.  Parsley — S.  Europe, 
Algeria,  Lebanon.  Rhubarb — Central  Asia.  Saffron — Asia  Minor. 
Sorghum — Tropical  Africa.  Spinach — Empire  of  Medes  and 
Persians.  Sugar  Cane — S.  Asia.  Tea — ^Indo-China.  Tobacco — 
America,  perhaps  Mexico,  Bolivia,  Venezuela.  Onion — Persia, 
Afghanistan.    Garlic — Europe. 

344.  Uncertainty  of  Origin.— The  original  habitat  of 
some  of  the  plants  is  uncertain,  owing  to  lack  of  definite 
precision  in  the  ancient  records.  Most  of  the  wild  plants 
and  many  of  the  cultivated  ones  antedate  definite  historic 
records.  Early  man,  ages  before  even  the  primitive  stages 
of  civilization,  was  instrumental  in  distributing  many  of 
the  plants.  It  should  be  noted  that  many  of  the  cultivated 
plants  are  varieties  produced  by  cultivation,  and  new 
varieties  are  constantly  appearing. 

The  attempt  to  trace  out  the  original  habitat  of  the  different 
plants  has  been  by  means  of  (1)  Geographical  Botany  which  deals 
with  the  distribution  of  plants,  supplemented  by  the  aid  of  (2) 
Archeology  and  Paleontology,  (3)  History,  (4)  Philology.  Con- 
clusions are  reached  only  after  carefully  sifting  the  data  from 
all  the  above  sources. 

The  table  given  above  was  compiled  mainly  from  the  follow- 
ing sources:    (1)  Origin  of  Cultivated  Plants  by  de  Candolle,  (2) 


GEOGRAPHY   OF   LIFE 


423 


Our  Plant  Immigrants  by  David  Fairchild.  (National  Geog.  Mag., 
April,  1906.)  (3)  Cyclopedia  of  American  Horticulture  by  L.  S. 
Bailey. 

With  the  aid  of  the  United  States  Census  report  on  agricul- 
tural products,  the  student  should  now  plot  on  a  map  of  the 
United  States  the  corn  belt,  wheat,  cotton,  sugar,  and  rice  belts, 
and  endeavor  with  the  aid  of  the  teacher  to  give  geographic  rea- 
sons for  their  location.  It  is  not  a  coincidence  that  corn  is  the 
chief  product  in  one  locality,  rice  in  another,  tobacco  in  another, 
etc. 

FORESTS 

One  of  the  most  important  topics  now  before  the  Amer- 
ican people  is  that  of  the  present,  past  and  probable  future 
conditions  of  the  forests.  When  this  country  was  first 
settled  by  white  men,  a  large  part  of  it  was  covered  by 
dense  forests.     The  early  settlers  chopped  down  the  trees 


Tig.   281.     Log  jam  at  Glens  Falls,   N.  f.     This  is  one  of  the  ways  in  which 
our  forests  are  disappearing.      (Natl.  Geog.  Mag.) 


424 


PHYSICAL    GEOGRAPHY 


and  burned  them  in  order  to  clear  the  land  for  their  farms. 
Later  the  lumbermen  cut  the  trees  by  millions  to  furnish 
the  lumber  to  build  the  cities,  villages,  factories,  etc.  So 
rapidly  has  cutting  of  the  forests  been  carried  on  that  ex- 
tensive areas  are  now  bare  and  barren  wastes,  for  the 
destruction  begun  by  the  chopper  has  been  completed  by 
the  fires.     (Fig.  282.) 

345.  Effects  of  Forest  Destruction.— On  most  of  the 
plateau,  plain,  and  valley  areas,  after  the  cutting  of  the 
forest  the  land  was  brought  under  cultivation,  and  is  now 


io.   -:-.      ;.urth   Sugar  Loaf  Mountain,  N.  H. 
a  barreu  waste. 


Once  heavfly  timbered,  now 


covered  with  prosperous  farms,  but  in  the  rocky  portions 
of  the  mountainous  and  hill  country,  the  soil  is  so  thin  or 
so  poor  that  it  cannot  be  cultivated,  and  the  result  is  that 
unproductive  and  unsightly  barren  waste  areas  now  mark 
the  sites  of  former  stately  forests. 


GEOGRAPHY    OF    LIFE 


425 


In  the  virgin  forests  there  was  an  accumulation  of  de- 
caying vegetation  that  furnished  a  rich  soil  for  the  forest 
trees.  The  fires  following  the  cutting  of  the  trees  burned 
up  the  vegetable  mould  leaving  bare  rocks  in  place  of  the 
former  deep,  rich  carpet  of  moss  and  shrubs. 

The  vegetable  carpet  of  the  forest  acts  like  a  great 
sponge  which  absorbs  and  holds  the  rainfall,  which  serves 
to  keep  the  area  moist  in  the  rainless  season.  The  destruc- 
tion of  the  vegetable  sponge  causes  most  of  the  rainfall  to 


Fig.  283.  Vegetation  on  the  surface  of  a  forest  acts  like  a  great  sponge,  pre- 
venting the  rapid  run-off  of  the  rainfall.  Yellow  pine  forest  in  the  Sierra 
Nevada  Mountains. 

run  over  the  rock  surface,  and  thus  wash  into  the  streams 
and  carry  away  the  residue  left  by  the  fire  and  drought. 
The  absence  of  the  forest  permits  more  of  the  rainfall 


426 


PHYSICAL    GEOGRAPHY 


to  run  directly  into  the  streams  producing  great  floods, 
which  means  destruction  to  property  in  the  valleys  and 
great  decrease  in  farm  products  from  drought  in  the  dry 
seasons.     (See  fig.  284.) 


M 

Bj^jfg^' 

f^ 

1 

B 

m 

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1 

% 

1 

i^^H 

r  ^  M. 

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hi 

mm  ' 

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m  M  f 

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V 

A. 

*'<!^^S^H 

Fig.   284.     The  destruction  of  the  forest  permits  the  rapid  erosion  of  the  soil 
by  heavy  rains.     Near  Marion,  N.  0.     (U.  S.  Geol.  Survey.) 


GEOGRAPHY    OF    LIFE 


427 


The  preservation  and  protection  of  forest  areas  on  the 
hills,  mountains,  and  plateaus  are  of  vital  importance  to 
the  prosperity  of  the  farms  in  the  valleys  as  well  as  the 
farms  on  the  upland. 

The  chief  products  obtained  from  the  great  forests  are 
lumber,  wood  pulp,  bark  for  tanning,  pitch,  tar,  and  tur- 


FiG.  285.  Forest  preserves  (in  black)  in  the  Western  United  States  in 
1902.  Some  have  been  added  since  that  time.  The  shaded  areas  are 
Indian  reservations.      (U.   S.   Geol.    Survey.) 


pentine.  The  smaller  trees  furnish  telegraph  and  tele- 
phone poles,  ties  for  railways,  pulp  for  paper  and  wood 
for  charcoal.  All  of  these  materials  are  necessary  in  our 
great  commercial  industries,  but  the  method  of  obtaining 
them  has  been  wasteful  and  extravagant  in  the  extreme. 


428 


PHYSICAL    GEOGRAPHY 


Now,  when  it  is  almost  too  late  to  remedy  it,  we  are  begin- 
ning to  realize  that  these  products  could  have  been  obtained 
without  the  enormous  waste. 

State  and  National  Legislatures  have  at  last  been  aroused  to 
the  importance  of  preserving  or  conserving  the  remnants  of  our 
former  great  forests  for  the  welfare,  not  only  of  the  future  gen- 


^^1^^^ 

W  ^^^^L            ,^^B^B^^^^^BBggjjJ^WBiiKL 

M 

^^F^^^Vfl^^^HM^r]|% 

^^^fep^jfc%!o 

^H^^^          ^^'wT           #• 

^7  ^T/^^^^£4^1r^  .      f 

'X>  / '^^^!k 

\^iLj 

Fig.  286.  Forest  areas  (in  black)  on  the  Western  United  States  in  1902. 
Part  of  the  area  has  since  been  deforested  by  lumbermen  and  fires. 
Shaded  areas  contain  a  scanty  growth  of  small  trees.  (U.  S.  Oeol. 
Survey. ) 


GEOGRAPHY    OF    LIFE 


429 


erations,  but  of  the  present  as  well.  We  now  have  considerable 
areas  of  forest  land  owned  and  controlled  by  the  nation  and  the 
different  states,  from  which  the  lumber  and  other  products  of 
the  forest  will  be  obtained,  without  the  destruction  of  the  forest. 
The  accompanying  map  shows  the  location  of  the  present  forest 
preserves,  and  it  is  to  be  hoped  that  these  preservations  will 
increase  in  size  and  number  from  year  to  year.  Schools  of  For- 
estry have  already  been  established  for  the  training  of  men  to 
properly  care  for  these  forests.     (Fig.  288.) 


Fig.    287.      Sequoia    Gigantea,    Mariposa    Grove    of   Big   Trees,    Cal. 
Pig.  278.) 


(See    also 


The  principal  districts  of  the  United  States  from  which  enor- 
mous quantities  of  lumber  have  been  obtained,  and  which  still 
contain  forest  remnants  of  great  value  are — 

I.  New  England,  including  Maine,  New  Hampshire,  and 
Vermont,  with  their  great  forests  of  pine,  hemlock, 
spruce,  and  cedar,  and  smaller  forests  of  oak,  maple, 
birch,  elm,  etc. 

II.  The  Adirondacks,  with  its  pine  and  spruce. 

III.  The  Great  Lake  region,  especially  northern  Michigan 
and  Wisconsin,  rich  in  pine  and  hemlock. 


430 


PHYSICAL    GEOGKAPHY 


Fig.   288.     I'orest  rangers  at  work  m  the  Texas  pine  forests.      (W.  L.  Bray.) 

IV.  The  Alleghany  Mountains  and  plateau  with  white  pine, 
hemlock,  hardwood  in  the  north,  and  yellow  pine  and 
hardwood  in  the  south. 

V.  The  Gulf  region  with  yellow  pine,  cedar,  cypress  and 
oak. 

VI.  The  Rocky  Mountain  region  with  bull  pine  and  spruce. 

VII.  The  Pacific  region,  the  richest  of  all  at  present,  with  its 
great  forests  of  redwood,  pine,  hemlock,  spruce,  and 
cedar.     (Figs.  278,  286  and  287.) 

ANIMAL   GEOGRAPHY 


346.  Zoological  Provinces  and  Faunas.— In  general, 
animals  have  a  greater  freedom  of  movement  than  plants. 
The  fact  that  they  have  the  power  of  shifting  quickly  from 


GEOGRAPHY    OF    LIFE  431 

place  to  place  makes  them  less  dependent  on  their  surround- 
ings. Most  of  the  forms  of  animal  life  can  by  their  power 
of  movement  escape  many  dangers,  such  as  frost,  fire  and 
floods,  that  destroy  plant  life. 

A  rabbit  may  nip  off  the  plant,  which  has  no  way  of 
escape  or  redress^  but  when  the  wolf  attempts  to  eat  the 
rabbit,  the  laUei  may  escape  by  flight,  and  the  former  may 
move  to  another  locality  and  seek  other  food.  However, 
there  are  more  or  less  well  defined  boundaries  beyond  which 
neither  the  rabbit  nor  the  wolf  is  likely  to  go.  The  area 
over  which  an  aggregate  of  associated  animals  wander  and 
struggle  for  existence  is  called  a  zoological  province.  All 
the  varieties  of  animals  which  characterize  a  zoological 
province  constitute  its  fauna. 

The  boundaries  limiting  these  provinces  are  not  always 
sharply  drawn.  There  is  usually  a  mingling  of  the  faunas 
of  adjoining  provinces  except  where  separated  by  some 
natural  feature  producing  an  abrupt  change  of  environ- 
ment. This  obstacle  to  the  spread  of  life  is  called  a  har- 
rier. 

The  boundaries  or  barriers  that  restrain  the  animals  to 
certain  provinces  or  areas  are  similar  in  many  ways  to  those 
which  control  the  spread  of  plants.  All  barriers  are  rela- 
tive and  not  insurmountable.  Mountains,  deserts,  the 
ocean,  changes  of  temperature,  the  relative  abundance  of 
other  life  in  the  form  of  food,  shelter,  or  enemies  are  all 
barriers  of  much  importance  in  the  study  of  the  distribu- 
tion of  animals  as  well  as  plants. 

As  with  the  plants,  so  with  the  animals;  the  ocean  forms  one 
of  the  most  important  of  all  barriers  to  land  forms  and  is  the 
chief  one  in  the  following  broad  classification  of  the  surface  of 
the  earth  into  zoological  regions: 

1.  North  America — including  North  America  as  far  south  as 
the  Isthmus  of  Tehuantepec.  Its  fauna  is  very  similar  to  that 
of  the  Eurasian  region,  and  they  have  more  species  in  common 


432 


PHYSICAL    GEOGRAPHY 


GEOGRAPHY    OF    LIFE  433 

than  any  of  the  other  provinces.  Among  the  forms  peculiar  to 
it  are  the  American  bison,  the  musk  ox,  the  Rocky  Mountain 
goat.  The  monkeys,  horses,  and  swine  do  not  exist  here  as  in- 
digenous forms. 

2.  Eurasian,  including  Europe,  Africa,  as  far  south  as  the 
Sahara,  and  Asia  north  of  the  Himalayas.  Here  are  large  num- 
bers of  carnivorous  animals,  such  as  the  wolf  and  wildcat,  to- 
gether with  the  reindeer,  camel,  and  many  varieties  of  wild  sheep 
and  goats.     The  monkey  tribe  is  entirely  absent. 

3.  South  American,  including  South  America,  the  West  Indies, 
Central  America,  and  southern  Mexico.  The  characteristic  forms 
are  the  tapir,  ant-eater,  sloth,  llama,  monkey,  and  the  condor  and 
rhea  among  the  birds.  Equally  characteristic  is  the  absence  of 
such  representative  families  as  oxen,  horses,  elephants,  anthro- 
poid apes,  and  moles. 

4.  African,  including  Africa  south  of  the  Sahara,  southern 
Arabia,  and  Madagascar.  It  has  the  greatest  number  of  species 
of  all  the  provinces,  and  here  the  ungulates  (hoofed  mammals) 
reach  their  greatest  development,  over  150  species  of  this  group 
being  known.  Well-known  African  animals  are  the  giraffe,  hip- 
popotamus, gorilla,  aebra,  ostrich,  and  lion.  The  two  latter  are 
characteristic  but  not  really  endemic,  that  is,  limited  to  this 
province.     The  most  notable  absences  are  the  bear  and  deer. 

5.  Oriental,  southern  Asia  and  the  islands  of  the  East  Indies 
east  to  the  Australian  region.  The  province  has  many  types  con- 
necting it  to  both  the  African  and  Eurasian  regions.  Species 
peculiar  to  it  are  the  tiger  (also  in  Eurasian),  orang-outang, 
jungle  bear,  tapir  and  several  species  of  antelopes. 

6.  Australian,  including  Australia,  New  Zealand  (sometimes 
made  a  sub  province).  New  Guinea,  and  other  smaller  islands. 
It  is  characterized  by  the  extreme  abundance  of  marsupials, 
typified  by  the  kangaroo,  animals  which  carry  the  young  in  a 
pouch,  the  very  peculiar  duckbill,  and  the  almost  complete  ab- 
sence of  all  five  higher  orders  of  terrestrial  mammals  from  the 
apes  to  the  ant-eaters.  The  emu,  cassowary,  and  lyre-birds  char- 
acterize the  bird  life. 

347.  Fresh  Water  Life.— While  some  animals  are  able 
to  exist  in  both  fresh  and  salt  water,  yet  the  faunas  are  so 
distinct  as  to  warrant  the  separate  consideration  of  fresh- 
water forms. 


434  PHYSICAL    GEOGRAPHY 

The  fresh  water  animals,  especially  those  inhabiting  the 
larger  lakes  are  divided  into  faunas,  much  as  in  the  ocean. 
The  forms  of  the  surface,  the  shores  and  the  bottom  are 
usually  here  quite  distinct.  In  the  rivers  the  distinction 
is  less  sharply  marked,  while  in  the  ponds  and  brooks  it 
cannot  be  drawn. 

These  fauna  include  amphibians,  larval  and  adult,  many 
varieties  of  fish,  the  larvae  of  many  types  of  insects,  and 
some  in  the  adult  stages,  many  Crustacea,  many  worms, 
one  variety  of  fresh-water  sponge,  and  other  lower  forms 
of  life.  There  is  great  variation  in  these  species,  for  the 
conditions  of  a  fresh  water  existence  vary  greatly  from 
the  turbulent  brook  to  the  majestic  lake. 

The.  transition  belt  at  the  mouths  of  rivers  emptying 
into  the  ocean,  where  the  water  is  brackish,  is  hostile  to 
most  forms  of  life,  both  of  salt  and  fresh  water,  and  has  a 
fauna  peculiar  to  itself. 

348.  Oceanic  Life.— The  animal  life  of  the  ocean  is 
wonderfully  varied  and,  to  the  interested  observer,  full  of 
beauty.  As  exposed  on  the  shores  between  tides,  brought 
to  the  surface  by  the  dredge  and  seine,  or  revealed  by  the 
water  telescope,  the  myriad  forms  entrance  the  beholder 
with  their  profusion  and  tempt  him  to  farther  study. 

The  distribution  of  oceanic  faunas,  while  governed  by 
the  general  principles  previously  outlined,  is  dependent 
upon  special  conditions  which  must  be  noticed. 

The  temperature  of  the  water  is  the  most  important 
factor  in  the  distribution  of  marine  animals.  It  is  de- 
pendent upon  three  conditions,  latitude,  ocean  currents 
and  depth. 

In  general  the  temperature  of  the  surface  oceanic  waters  Is 
higher  at  the  equator  and  progressively  diminishes  toward  the 
poles.  This  is,  however,  modified  by  the  ocean  currents  which 
carry  immense  volumes  of  warm  water  into  regions  where  the 


GEOGRAPHY    OF    LIFE  435 

waters  bordering  the  current  are  considerably  colder.  The 
abrupt  change  thus  produced  is  one  of  the  important  barriers 
existing  in  the  ocean.  Species  may  be  able  to  extend  their  range 
widely  where  the  change  in  temperature  is  gradual,  but  these 
same  species  often  cannot  endure  the  abrupt  change  of  passing 
from  warm  to  cool  water  or  the  contrary.  This  is  especially  true 
of  the  eggs  of  many  of  the  molluscs.  Hence,  ocean  currents  often 
form  important  boundaries  to  oceanic  provinces.  Similarly  depth 
is  an  important  condition,  for  it  directly  affects  temperature,  as 
the  farther  from  the  surface  the  cooler  the  water.  It  is  closely 
related  to  the  effect  of  altitude  upon  the  land,  a  few  thousand 
feet  vertically  causing  greater  changes  in  the  faunas  than  many 
hundreds  of  miles  of  latitude.  So  important  is  its  effect  that  it 
is  used  as  the  basis  upon  which  oceanic  life  is  classified,  and  the 
following  four  great  life  zones  are  marked  off  by  it. 

Littoral  Life,  that  of  the  shore,  is  the  best  known  to 
the  ordinary  observer  of  any  of  the  oceanic  zones.  On 
the  beach  at  low  tide  among  the  seaweed  are  found  many 
interesting  forms,  the  shore  birds,  many  varieties  of  mol- 
luscs with  .their  oddly  shaped  and  often  brightly  colored 
shells,  sand  worms,  starfish,  sea  urchins,  in  fact,  represen- 
tatives of  almost  every  known  class  of  animals.  It  is 
small  wonder  that  men,  from  the  ancient  Greeks  to  the 
modern  scientist,  have  strongly  believed  that  where  land 
and  sea  meet,  life  originated. 

The  conditions  of  life  here  are  greatly  varied.  With 
the  ebb  and  flow  of  the  tide,  the  dashing  of  the  waves,  the 
winds,  the  sunlight,  what  more  invigorating  environment 
can  be  conceived? 

Intermediate  Life,  (sometimes  included  in  Littoral)  in- 
cludes the  life  at  moderate  depths,  ranging  from  low 
water  level  to  a  depth  approximating  500  fathoms.  It  is 
not  sharply  separated  from  the  Littoral  and  is  sometimes 
included  in  it.  It  is  the  zone  of  seaweeds  and  corals,  and 
here  marine  life  reaches  its  maximum  both  in  number  and 
variety  of  forms.     Here  flourish  molluscs  of  many  types, 


436 


PHYSICAL    GEOGRAPHY 


including  the  common  clam  and  oyster,  corals,  sea  anem- 
ones, sea  cucumbers,   crinoids  or  sea  lilies,   sponges  and 


Fia.  290.  Skate,  one  of  the  odd  but  rather  abundant  forms  of  fish  in  the 
intermediate  zone  of  marine  life.  Cape  Flattery  Bank,  Washington. 
(U.  S.  Fish  Commission.) 


GEOGRAPHY    OF    LIFE 


437 


many  lower  forms,  along  with  lobsters,  crabs,  and  the  vast 
multitude  of  fishes.  Here  life  conditions,  while  more  uni- 
form than  those  of  the  Littoral  zone,  have  not  reached  the 
unvarying  monotony  of  the  great  deeps.  Food  is  abun- 
dant, the  depth  is  not  too  great  for  sunlight  to  penetrate, 
and  the  waters  are  comparatively  warm  and  undisturbed. 

Abyssal  life.  The  boundary  between  the  preceding 
zone  and  the  abyssal,  or  deep  sea,  is  not  sharply  defined, 
many  species  passing  far  to  each  side  of  the  arbitrary 


Pig.  291.  One  of  the  deep  sea  fishes.  They  live  in  perpetual  darkness  in  the 
cold  waters  on  the  floor  of  the  ocean  basins.  Taken  from  a  depth  of 
several  thousand  feet.     (Smith.  Inst.) 


438 


PHYSICAL    GEOGRAPHY 


depth  taken  as  the  dividing  line.  Conditions  of  life  here 
are  extremely  uniform,  no  light,  a  uniform  temperature 
approximating  34  degrees,  great  pressure,  little  motion  of 
water,  and  a  high  percentage  of  oxygen.  Few  plants  live 
here,  and  the  animals  are  carnivorous,  feeding  upon  each 
other  and  animal  remains,  which  slowly  sink  down  from 
the  surface. 


Fig.   292.      One  of  the  odd-shaped  forms  of  life  from  the  bottom  of  the  deep 
sea.      (Smith.    Inst.) 


Abyssal  animals  are  much  the  same  the  world  over,  and  in 
spite  of  the  seeming  adverse  conditions  under  which  they  live, 
the  life  is  quite  varied,  including  all  the  main  types  from  the 
fishes  down.  Many  of  the  forms  are  extremely  odd  and  curious. 
Knowledge  of  deep-sea  life  has  been  greatly  extended  during  the 
recent  years  by  the  use  of  the  dredge.     (Figs.  291  and  292.) 

Pelagic  life  includes  those  forms  which  habitually  live 
on  the  surface  of  the  open  sea  or  at  moderate  depths  below 
it.  Here  under  the  favorable  condition  of  abundant  sun- 
light and  moisture,  and  with  many  minute  forms  which 
furnish  food  to  the  larger  animals,  a  rich  and  varied  life 
is  found.     Whales,   many   forms  of   fish,   pteropods   and 


GEOGRAPHY    OF    LIFE  439 

cephalopods  among  molluscs,  crustaceans,  and  vast  num- 
bers of  lower  forms  such  as  the  Portuguese  man-of-war, 
jelly-fish,  infusoria  and  radiolaria  are  all  pelagic  in  their 
habits. 

Between  the  surface  zone  with  its  rich  fauna  and  the  ocean 
bottom  with  its  scanty  fauna  is  the  great  body  of  oceanic  waters 
which,  so  far  as  our  present  knowledge  shows,  is  almost  devoid 
of  life. 

THE    GEOGRAPHY    OF    MAN 

349.  Distribution  of  Mankind.— At  the  World's  Fair 
in  St.  Louis  one  could  see  men  from  all  the  continents  and 
many  of  the  larger  islands  of  the  world.  There  was  a 
great  diversity  in  color,  size,  and  other  physical  features. 
Are  these  people  all  from  one  parent  family,  and  if  so, 
where  was  the  home  of  that  family,  and  how  came  this 
great  diversity  in  racial  characteristics? 

The  original  habitat  of  man  is  thought  to  be  in  western 
Asia,  from  whence  descendants  migrated  in  different  direc- 
tions. Probably  the  principal  factor  in  the  variations  in 
color,  size,  facial  features,  and  mental  development  was 
geographical  environment.  After  a  long  period  of  time 
the  slow  changes  finally  resulted  in  a  great  many  races 
and  tribes  which  are  sometimes  grouped  in  the  following 
four  races : 

1.  The  Ethiopian   or  black  race  Number— 173,000,000 

2.  The  Mongolian  or  yellow  race  *'  540,000,000 

3.  The  American  or  red  race  **  22,170,000 

4.  The  Caucasian  or  white  race  '*  770,000,000 


Total 1,505,170,000 

The  original  habitat  of  the  Ethiopian  race  was  Africa 
south  of  the  Sahara,  Madagascar,  and  many  of  the  East 


440  PHYSICAL    GEOGRAPHY 

Indies.  The  second  probably  started  from  the  Tibetan 
table-land.  The  third  occupied  the  new  world,  America. 
The  fourth  race  probably  started  in  North  Africa.  From 
the  original  habitat  these  races  have  spread  over  the  world 
and  are  now  mingled  on  all  the  continents. 

For  many  centuries,  the  Eed  man  held  undisputed 
sway  in  what  is  now  the  United  States.  About  four  cen- 
turies ago  the  white  man  first  came  in  small  numbers,  then 
in  larger  numbers,  and  later  he  brought  the  Black  man 
first  as  a  slave.  The  yellow  man  found  his  way  across  the 
Pacific  Ocean  and  entered  our  western  border.  We  now 
have  all  four  races  in  large  numbers.  So  on  all  the  con- 
tinents there  is  a  commingling  of  different  races. 

It  must  be  remembered  that  the  above  classification  is  sn 
arbitrary  one,  and  only  one  of  many  attempts  to  classify  the 
human  family.  Each  of  the  divisions  contains  many  classes  and 
tribes,  each  with  its  own  characteristics  of  physical  and  mental 
traits.  There  are  distinctions  in  size  and  shape  of  the  skull, 
color  and  texture  of  the  hair,  language,  and  above  all,  mental 
development.  A  much  more  elaborate  classification  is  given  in 
the  Standard  Dictionary  under  Man. 

The  Mongolians  have  probably  an  older  fairly  authentic  his- 
tory than  any  of  the  others,  but  as  far  back  as  their  history  ex- 
tends, the  Chinese  and  the  Japanese  branches  are  separate. 
They  have  retained  their  present  national  characteristics  through 
a  longer  period  of  time  with  less  changes  than  any  of  the  other 
nations. 

The  peoples  that  have  passed  through  the  greatest  changes 
and  most  rapid  development  in  civilization  are  some  of  the 
branches  of  the  Caucasian  race.  In  none  of  the  nations,  how- 
ever, does  authentic  history  extend  back  to  the  beginning  of  the 
race.  That  the  different  races  are  branches  of  one  common  fam- 
ily is  an  inference  or  deduction  based  on  a  study  of  the  whole 
human  family  in  their  physical  and  mental  characteristics,  and 
relations  to  each  other  and  the  other  animal  forms.  The  origin 
of  the  human  family  is  involved  in  obscurity. 

350.    Influence  of  Geography  on  Man.— While  man  by 


GEOGRAPHY    OF    LIFE  441 

his  ingenuity  has  prevailed  over  the  forces  of  Nature  in 
many  ways,  yet  it  still  remains  true  that  he  is  greatly  in- 
fluenced by  his  geographic  surroundings.  The  climate, 
topography,  proximity  to  the  sea,  all  wield  a  wonderful 
influence  over  man. 

351.  Climate.— All  the  great  nations  of  the  world  have 
had  their  rise  and  growth  in  the  temperate  climate.  This 
is  not  a  coincidence.  Man  may  carry  civilization  into  cold 
and  hot  climates  and  may  foster  it  for  a  time,  but  the  fact 
remains  that  the  development  of  the  great  civilized  nations 
has  been  in  the  temperate  zone. 

The  continued  heat  of  the  tropics  tends  to  make  one 
languid  and  lacking  in  enterprise.  The  warm  climate  re- 
quires little  clothing  or  shelter  to  protect  one  from  the  ele- 
ments. The  great  abundance  of  tropical  fruit  makes  the 
food  supply  an  easy  problem.  Thus  the  great  incentive 
to  provide  food,  clothing,  and  shelter  which  arouses  man  to 
his  best  eft'orts  in  a  cooler  climate  is  lacking  in  the  warm 
tropics. 

In  the  polar  regions  the  cold  is  so  intense  and  long  con- 
tinued that  it  is  a  perpetual  struggle  for  existence ;  hence, 
one  does  not  find  the  opportunity  to  cultivate  the  mind 
and  surround  himself  with  the  luxuries  and  comforts  of 
modern  civilization. 

In  the  temperate  region  the  cold  winters  rouse  man  to 
exertion  to  provide  f^od,  clothing  and  shelter,  and  yet  the 
cold  is  not  so  severe  as  to  dwarf  his  energies  or  prevent  the 
development  of  his  mental  and  physical  powers. 

352.  Influence  of  Topographic  Forms.— As  already 
stated  the  surface  features  greatly  influence  the  distribu- 
tion of  the  population,  as  well  as  the  occupations  of  the 
people  and  many  of  their  customs  and  habits. 

The  most  densely  populated  areas  are  generally  on  the 
plains  because  facilities  for  travel  and  transportation  are 


442  PHYSICAL    GEOGRAPHY 

there  superior  and  favor  the  commercial  and  manufactur- 
ing industries.  Such  conditions  also  favor  agriculture 
and  hence  abundant  food  supply. 

Life  in  the  high  mountains  tends  toward  the  isolation 
of  groups  of  people  and  the  development  and  perpetuation 
of  local  customs.  There  is  little  communication  and  in- 
tercourse with  outside  people.     In  the  absence  of  mineral 


Fig.   293.     Farm  house  in  the  Boston  Mountains.    In  many  mountainous 
districts  farming  is  not  a  profitable  industry. 

products  or  large  forests  the  mountain  people  are  liable  to 
be  poor  and  live  a  very  simple  life  with  few  luxuries.  In 
the  primitive  and  pioneer  stagas  the  people  in  the  moun- 
tains depend  largely  for  support  upon  hunting  and  fishing. 
The  charm  and  the  freedom  of  such  a  life  overcomes  in 
large  measure  any  desire  for  so-called  luxuries,  and,  hence, 
any  incentive  or  opportunity  for  wealth.     When  the  moun- 


GEOGRAPHY    OF    LIFE  443 

tain  people  are  dependent  upon  agriculture,  the  barren 
soil  and  rough  surface  prevents  any  profit  more  than  mere 
subsistence;  and,  hence,  the  hard  struggle  for  food  and 
shelter  hinders,  if  it  does  not  prevent  advancement  in  cul- 
ture, learning  and  conveniences  of  civilization.  (Fig.  293.) 
353.  Proximity  to  the  sea  generally  favors  easy  com- 
munication with  other  nations  and  countries,  and  hence 
fosters  the  commercial  spirit  which  results  in  wealth  and 
cosmopolitan  ideas.  In  past  centuries  many  of  the  sea- 
faring nations  were  war-faring  as  well  and  conquered  by 
might  where  at  the  present  day  the  battles  are  fought 
more  on  manufacturing  and  commercial  lines. 

A  few  examples  will  best  illustrate  the  influence  of  geo- 
graphic conditions  on  man. 

The  Eskimo  gives  all  his  time  and  energy  to  the  chase. 
He  has  no  chance  to  raise  vegetables,  even  if  he  desired.  Hav- 
ing but  a  limited  supply  of  fuel  he  learns  to  depend  largely  on 
the  conservation  of  his  own  bodily  heat  for  warmth,  so  he 
dresses  in  furs,  eats  fat  and  lives  in  ice  houses.  His  life  is  not 
devoid  of  adventure,  but  there  is  little  incentive  to  advancement, 
and  the  Eskimos  to-day  are  probably  no  better  off  than  their 
ancestors  were  centuries  ago. 

The  Pigmies  of  the  African  forest  live  in  a  rude  shelter  of 
bushes  quickly  constructed  and,  hence,  it  is  no  great  hardship 
to  leave  it  and  migrate  to  a  distant  part  of  the  forest.  They  have 
no  agriculture  and  live  on  nuts  and  wild  animals  which  they  cap- 
ture in  snares  and  pits.  Having  no  reserve  supply  of  food  they 
are  frequently  subject  to  hunger  and  sometimes  starvation. 
There  is  no  development  of  the  mental  faculties,  and  they  remain 
but  little  superior  intellectually  to  the  animals  which  they  pursue. 

Emigrants  from  Europe,  scarcely  three  centuries  ago,  entered 
the  present  area  of  the  United  States.  They  have  cut  down  the 
forests,  cultivated  the  soil,  built  cities  and  factories,  extended 
steam  railways  and  electric  lines  far  and  wide  over  the  land. 
Many  boats  ply  the  inland  waters  and  hundreds  of  vessels  sail 
to  and  from  foreign  lands.  Telegraph  lines  extend  to  distant 
parts  of  the  earth.  One  may  read  in  the  evening  papers  an  ac- 
count of  any  or  all  of  the  important  events  that  have  happened 


444  PHYSICAL    GEOGRAPHY 

anywhere  in  the  world  during  the  day.  If  one  desires  he  may 
without  leaving  his  chair  talk  by  telephone  with  any  of  his  friends 
within  a  radius  of  several  hundred  miles.  He  has  in  his  service 
many  of  the  varied  products  of  the  world;  fruits  of  the  field, 
garden,  mine,  and  factory  are  at  his  command.  What  a  contrast 
is  this  life  with  that  of  the  Eskimo  or  the  African  Pigmy! 

The  difference  in  habit,  customs,  civilization,  and  develop- 
ment of  these  peoples  is  not  entirely  due  directly  to  climate,  but 
partially  to  racial  differences.  The  North  American  Indian  that 
was  here  before  the  European,  was  and  is  yet,  in  his  natural  state, 
as  far  below  the  white  in  civilization  as  he  is  superior  to  the 
Pigmy.  How  far  these  racial  differences  are  due  to  climatic  con- 
ditions in  past  ages  is  a  subject  worthy  of  consideration. 

The  influence  of  geography  on  the  migrations  of  man,  in  the 
founding  of  cities,  in  the  construction  of  highways,  has  been  sug- 
gested at  different  places  in  the  preceding  pages  and  will  be  con- 
stantly suggested  to  the  student  of  history  and  geography  in  all 
his  study  and  travel. 

The  migrations  westward  from  the  early  settlement  at  Phil- 
adelphia first  spread  out  in  the  Chester — "the  Little  Valley" — 
and  later  in  the  "Great  Valley"  because  in  these  valleys  was  a 
rich  limestone  soil  more  productive  and  more  easily  tilled  than 
that  on  the  bordering  hills.  The  further  migrations  were 
through  the  water  gaps  into  the  other  valleys  of  the  Alleghany 
Mountains,  where  the  people  lingered  long  before  making  the 
difficult  and  dangerous  journey  up  and  over  the  rocky  forested 
plateau  extending  west  to  the  Ohio  region.  Besides  the  great 
diflSculty  and  danger  in  traveling,  the  climate  of  the  plateau  is 
more  severe  and  the  soil  less  fertile  than  in  the  sheltered  valleys 
and  coves.  Hence  in  the  valleys  they  lingered  until  the  French 
were  pouring  into  the  Ohio  Valley  by  ascending  the  St.  Lawrence 
and  crossing  the  Great  Lakes  and  descending  the  tributaries  of 
the  Ohio.     (See  fig.  294.) 

Pittsburg  was  a  strategic  point  in  the  early  colonial  days  and 
as  Fort  Duquesne  and  Fort  Pitt,  it  was  the  scene  of  bloody  con- 
flicts between  the  nations.  The  early  settlers  knew  nothing  of 
the  great  coal  beds  and  the  deposits  of  oil  and  gas  that  have  been 
so  instrumental  in  making  this  one  of  the  great  manufacturing 
cities  of  the  world,  but  they  could  see  its  great  advantages  as  a 
commercial  center,  and  hence,  the  feverish  haste  of  the  French 
to-  get  possession,  and  of  the  English  to   dispossess  them. 


GEOGRAPHY    OF    LIFE 


445 


446  PHYSICAL    GEOGRAPHY 

It  is  only  necessary  to  study  the  geographic  location  and 
surroundings  of  New  York,  Boston,  Chicago,  Buffalo,  and  the 
other  great  cities  to  see  that  their  growth  has  been  governed  by 
geographic  features,  which  frequently  were  not  perceived  by  the 
people  at  the  time,  but  which  governed  them,  nevertheless. 
The  student  from  previous  reading,  study  and  observation  should 
put  in  writing  the  geographic  reasons  for  the  location  and  growth 
of  our  great  cities.  These  should  be  compared  in  the  class-room 
and  supplemented  by  explanations  from  the  teacher. 

354.  Influence  of  Man  on  Geography.— Man  is  not  a 
mere  passive  agent.  While  he  has  been  influenced  in 
many  ways  by  his  geographic  surroundings,  he  has  had  a 
very  marked  influence  on  them  in  return.  As  evidence  of 
this  one  needs  but  to  compare  the  United  States  of  to-day 
with  its  condition  four  centuries  ago. 

A  large  part  of  the  dense  forests  has  been  destroyed. 
The  freshly  plowed  soil  exposed  to  the  rains  has  been 
washed  in  large  quantities  into  the  streams  and  carried  to 
or  toward  the  sea.  In  the  construction  of  cities,  highways, 
and  railways,  hills  have  been  cut  through,  sometimes  cut 
down,  valleys  and  lakes  have  been  filled  or  partly  filled. 
Streams  have  been  diverted  from  their  courses.  Great 
dams  or  lakes  have  been  constructed  in  some  places  and 
destroyed  in  others.  Many  of  the  wild  animals  have  been 
wholly  or  partly  destroyed  and  domestic  animals  have 
taken  their  place.  Orchards  have  replaced  the  forest  in 
part,  grains  and  vegetables  have  taken  the  place  of  the 
wild  plants  over  large  areas. 

Canals  have  been  dug  across  divides  connecting  differ- 
ent river  basins.  The  Chicago  Drainage  Canal  carries 
water  from  Lake  Michigan  into  the  Mississippi  River, 
water  that  under  natural  conditions  would  drain  through 
the  St.  Lawrence  River.  We  are  even  attempting  to  con- 
nect the  Atlantic  and  Pacific  Oceans  by  an  artificial  chan- 
nel. 


GEOGRAPHY   OF   LIFE  447 

The  steel  bands  of  the  railway  connect  the  oceans  at 
several  points  and  a  considerable  portion  of  the  intervening 
territory  is  covered  with  a  lacework  of  steel  rails  over 
which  millions  of  tons  of  material  are  being  shifted  from 
one  part  of  the  country  to  another,  and,  in  connection  with 
the  steamboats,  part  of  it  even  to  -distant  countries. 

Great  stone  quarries  in  many  places  have  left  holes  in 
place  of  hills.  Clay,  sand,  gravel,  marl,  and  ore  pits  are 
in  many  places  so  numerous  and  extensive  as  to  entirely 
change  the  surface  features  of  the  area. 

In  many  places  the  mountains  and  plateaus  are  bored 
and  tunnelled  by  numerous  excavations  to  an  extent  al- 
most beyond  belief.  Besides  the  large  mine  openings  man 
has  bored  thousands  of  deep  holes  through  which  have  been 
taken  vast  accumulations  of  oil,  and  gas. 

Elsewhere  through  artesian  wells  he  has  brought  the 
ground  water  to  the  surface  in  arid  areas  and  thus  added 
to  the  fertility  of  the  country.  In  other  localities  where 
there  was  too  much  water  and  the  land  was  swampy, 
malarial,  and  unproductive,  he  has  by  surface  or  sub-sur- 
face draining  made  it  dry,  healthful,  and  productive. 

REFERENCES 

Coulter,  J.  M.,  Plant  Relations,  D.  Appleton  &  Co. 

Clements,  Research  Methods  in  Ecology,  University  Pub.  Co., 
Lincoln,  Nebraska. 

Schimper,  Planzen  Geographie,  Jena,  Gustav  Fischer. 

Cowles,  The  Physiographic  Ecology  of  Chicago  and  Vicinity, 
Bot.  Gaz.  31,  73,  1901. 

Wolle,  Diatomaceae  of  N.  America  (Hid.  with  2,300  figs.) 
The   Comenius  Press,  Bethlehem,  Pa.,   1894. 

Bray,  Distribution  and  Adaptation  of  the  Vegetation  of  Texas, 
Bull.   82,   University  of  Texas. 

Lamson,  Scribner,  Grasses  as  Sand  and  Soil  Binders.  Year- 
book, U.  S.  Dept.  of  Agr.,  1894. 


448  PHYSICAL    GEOGRAPHY 

Hill,  Physical   Geography   of  the  Texas  Region,  U.  S.  Geol. 

Surv.   Topographic  Atlas. 
Hitchcock,  Methods  Used  in  Controlling  and  Reclaiming  Sand 

Dunes.     U.  S.  Dept.   of  Agr.,  Bur.  of  Plant  Industry, 

Bull.  No.  57. 
Lloyd  &  Tracy,  The  Insular  Flora  of  Miss,  and  La.,  Dept.  of 

Botany,  Columbia  University,  No.  174. 
Webber,  The  Water  Hyacinth  and  its  Relation  to  Navigation. 

U.  S.  Dept.  of  Agr.,  Bull.  No.  18. 
Allen — 1.     The     Geological     Distribution     of     Animals,     Bull. 

U.   S.  Geol.  and  Geog.   Surv.   of  the  Territories, .Vol. 

IV,  pp.  313  to  377. 
2     The  Geological  Dis.  of  N.  American  Mammals,  Bull. 

Am.  Mus.  Nat.  His.,  Vol.  V,  pp.  199-243. 
Heilprin,  Angelo,  The  Geog.  and  Geol.  Dist.  of  Animals.    Int. 

Sci.  Ser.,  London  and  New  York,  1897. 
Osborne,  The  Rise  of  the   Mammalia  in  N.  America,  N.  Y., 

1893. 
Merriam — 1.    The  Geog.  Dist.  of  Life  in  N.  America,  Proc. 

Biol.  Sur.,  Washington,  Vol.  VII,  1892. 

2.     Life-Zones  and  Crop-Zones  of  the  U.  S.,  Bull.  No. 

10,  Dept.  of  Agr.  Div.  of  Biol.  Surv. 


CHAPTER  XII 

PHYSIOGRAPHIC   REGIONS  OF  THE  UNITED 
STATES* 

The  area  of  the  United  States  is  so  large  and  diversified 
that  it  contains  numerous  examples  of  all  the  different 
physiographic  types  previously  described. 

The  entire  area  of  the  United  States  is  conveniently 
divided  for  study  into:  (1)  the  Eastern  or  Atlantic  region; 
(2)  the  Lake  region;  (3)  the  Central'  or  Mississippi 
region;  (4)  the  Southern  or  Gulf  region;  (5)  the  Western 
Interior  region  and  (6)   the  Pacific  region. 

355.  1.  The  Eastern  or  Atlantic  Region.— Under 
this  heading  is  included  the  eastern  part  of  the  United 
States  next  to  the  Atlantic  Ocean  but  not  confined  to  the 
area  that  drains  into  it.  The  southern  part  of  the  moun- 
tainous area  drains  westward  into  the  Mississippi,  yet 
physiographically  it  belongs  to  the  same  province  as  the 
northern  part  which  drains  eastward  into  the  Atlantic. 

A.  The  Atlantic  coastal  plain  is  the  part  bordering 
the  seashore  and  may  be  divided  into  three  portions;  (a) 
the  submarine  plain,  corresponding  to  the  part  of  the  con- 
tinental shelf  on  our  eastern  seaboard.     It  is  now  under 

*The  different  regions  may  be  studied  by  following  the  order  of  geo- 
graphical position,  that  is,  beginning  at  one  side,  as  the  east,  and  taking  up 
each  region  in  turn  across  the  country  to  the  west  side,  or  by  studying  all 
the  areas  of  the  same  feature  at  one  time,  as,  for  example,  all  the  moun- 
tains first,  then  the  plateaus  and  plains.  While  there  are  advantages  in  each 
method  the  author  favors  the  first.  However,  any  teacher  preferring  the 
second  method  can  readily  use  it  with  the  map  and  data  given.  In  either 
case  only  a  brief  outline  can  be  given  in  a  general  treatise  of  this  kind, 
and  the  student  will  find  it  advisable  to  take  up  a  few  of  the  areas  more 
thoroughly  by  utilizing  some '  of  the  reference  works  cited.  A  small  map 
showing  all  the  different  regions  should  be  made  by  the  student. 
29  449 


450 


PHYSICAL    GEOGRAPHY 


Fig.  295.  Photograph  of  Relief  map  of  part  of  Eastern  and  Central  United 
States.  (E.  E.  Howell.)  Trace  out  on  this  the  physiographic  regions 
mentioned  in  the  text. 


THE    PHYSIOGRAPHIC    REGIONS 


451 


the  sea  but  parts  of  it  have  been  land  area  at  times  in  the 
past  and  probably  will  be  in  the  future;  (b)  the  tidal 
flats  and  coastal  marshes  comprise  the  portions  of  the  coastal 
plain  that  are  at  least  partly  exposed  during  the  low  tide 
and  are  largely  covered  with  water  during  high  tide.     In 


*?  - 


"^l^'i^ 


Fig,  296.  View  on  the  low  plains  of  Central  Florida.  The  alligator  and 
rattlesnake  are  abundant  where  not  destroyed  by  man.  The  vegetation  is 
typical  of  the  swamp  areas.  Pine  forests  where  not  destroyed  occur  on 
the  more  elevated  portions.      (A.  M.  Reese.) 

some  places  they  are  covered  with  salt  water,  in  others 
brackish  water  (a  mixture  of  salt  and  fresh)  and  in  others 
by  fresh  water.  Some  of  the  fresh  water  swamp  areas  are 
above  high  tide  but  intimately  connected  with  the  tidal 
areas,  (c)  The  emerged  plains  include  portions  of  the 
coastal  plain  elevated  above  high  tide,  extending  back  in 
places  many  miles  from  the  sea.     They  are  now  covered 


452 


PHYSICAL    GEOGRAPHY 


THE    PHYSIOGRAPHIC    REGIONS  453 

with  layers  of  sand,  clay,  and  marl  that  were  deposited 
over  the  former  sea-bottom,  and  contain  a  great  deal  of 
valuable  farm  land.  In  places  the  tidal  fiats  are  absent 
and  the  emerged  plains  are  separated  from  the  submarine 
plain  by  the  shore  line.  What  is  the  relation  of  the  Pall 
Line  to  this  area?     (See  sec.  116.) 

B.  The  Piedmont-New  England  Plateau,  Inland  from 
the  sandy  coastal  plain  and  separating  it  from  the  moun- 
tains is  an  area  covered  with  hard,  crystalline  rocks  and 
having  a  hilly,  irregular  surface.     (See  fig.  297.) 

Since  it  lies  at  the  foot  of  the  higher  mountains  further 
west  it  is  called  i)ied-  (foot)  mont  (mountain).  Since  it 
is  elevated  above  the  more  recent  coastal  plain  so  promi- 
nently it  is  called  a  plateau.  It  varies  in  width  through 
the  Middle  and  Southern  states,  reaching  its  greatest  width 
in  New  England.  The  greater  part  of  the  piedmont  belt 
was  in  early  geological  times  covered  or  partly  covered 
with  mountains  which  during  long  ages  were  worn  down 
nearly  to  a  plain,  that  is,  to  a  peneplain.  The  entire  area 
was  elevated  again  and  the  streams  cut  many  deep  valleys 
into  the  uplifted  plain,  dissecting  it  into  a  complexity  of 
hills  and  valleys.  The  Fall  Line  separates  this  from  the 
coastal  plain.     (Figs.  194  and  297.) 

C.  The  Appalachian  Mountain  Area.  The  somewhat 
complex  mountainous  area  in  the  Eastern  United  States 
is  divided  into  four  regions  each  of  which  in  turn  has 
many  complexities.     (Fig.  295.) 

(a)  The  first  range  of  mountains  bordering  the  Pied- 
mont plateau  on  the  west  is  called  the  South  Mountain  in 
Pennsylvania  and  the  Blue  Ridge  in  Virginia  and  farther 
south.  It  is  a  very  irregular  range,  both  in  size  and  struc- 
ture. It  is  composed  in  part  of  very  old  rocks,  hard  and 
crystalline,  in  part  of  brown  sandstones  and  shales  of  more 
recent  age. 


454  PHYSICAL    GEOGRAPHY 

(b)  The  Great  Valley  of  the  Appalachians  is  the  great 
depression  that  separates  the  South  Mountain  Blue  Ridge 
from  the  ridges  of  the  Alleghany  Mountains  on  the  west. 
It  is  a  broken  diversified  area,  quite  hilly  in  places,  and 
very  different  from  an  ordinary  river  valley.  It  is  only 
when  we  consider  it  in  its  relation  to  the  mountains  on 
each  side  that  it  appears  as  a  valley.  Part  of  the  area  is 
underlain  by  limestone  and  the  other  part  mostly  by  shale. 
In  the  residual  clay  overlying  the  rocks  are  vast  quantities 
of  iron  ore  and  white  clay. 

The  great  Shenandoah  Valley  of  Virginia,  the  Cumber- 
land and  Lebanon  Valleys  in  Pennsylvania  lie  in  and  form 
part  of  the  Great  Valley  that  extends  from  Lake  Cham- 
plam,  New  York,  to  Alabama. 

(c)  Bordering  the  Great  Valley  on  the  west  are  the 
Alleghany  ridges  which  have  a  general  northeast-south- 
westerly trend.  The  ridges  have  a  somewhat  uniform 
height  and  are  quite  variable  in  length.  Some  extend  only 
a  few  miles,  some  a  hundred  miles  or  more.  In  places 
they  are  like  upturned  canoes,  the  "Canoe  Mountains,"  in 
other  places  they  twist  and  turn  in  different  directions. 
(See  figs.  238  to  242.) 

The  ridges  are  almost  all  composed  of  sandstone  which 
on  disintegration  gives  rise  to  a  quite  unproductive  sandy 
soil.  The  valleys  separating  the  ridges  have  generally  a 
limestone  soil  which  is  quite  fertile.  The  valleys  are 
decked  with  valuable  farms  and  the  ridges  formerly 
forest-clad  are  now  covered  only  with  rocks  and  bushes. 
Their  chief  products  at  present  are  pure  water,  pure  air, 
huckleberries  and  rattlesnakes,  of  which  the  first  and  third 
can  be  transported  to  the  towns  in  the  valleys  but  the  other 
two  can  be  best  enjoyed  by  a  trip  into  the  mountains. 

(d)  Immediately  west  of  the  Alleghany  ridges  is  the 
Appalachian  plateau,  including  the  Alleghany  plateau  on 


THE    PHYSIOGRAPHIC    REGIONS 


455 


the  north  and  the  Cumberland  plateau  on  the  south.  It  is 
bounded  on  the  east  by  a  steep  irregularly  notched  escarp- 
ment facing  the  narrow  valley  that  separates  it  from  the 
Alleghany  ridges,  and  it  slopes  westward  gradually  merg- 


FiG.  298.  Typical  view  on  the  Alleghany  Plateau.  Potomac  Valley,  near 
Chaffee,  Md.  (Md.  Geol.  Survey.)  The  plateau  is  covered  with  forests 
(where  not  destroyed)  and  parts  of  it  are  underlaid  by  beds  of  coal,  and 
pools  of  petroleum  and  natural  gas.  It  is  dissected  by  many  rivers  which 
flow  in  deep  valleys. 

ing  into  the  Mississippi  and  Lake  Plains.  It  is  deeply 
trenched  by  great  numbers  of  streams  so  that  in  many 
places  it  resembles  an  irregular  mountain  mass.  (See 
Charlestown,  W.  Va.,  topographic  sheet.)  There  are  a 
few  elevated  mountain  ridges  in  this  plateau  area,  such  as 
Chestnut  and  Laurel  Ridges  in  Western  Pennsylvania. 

The  Catskill  Mountains  form  the  northeastern  end  of 
the  Alleghany  plateau.     Many  of  the  more  rugged  portions 


456 


PHYSICAL   GEOGRAPHY 


THE    PHYSIOGRAPHIC    REGIONS  457 

further  south  are  locally  known  as  mountains,  but  like  the 
Catskills  they  are  mountains  of  circum-erosion.  The  Erie 
Canal  and  New  York  Central  Railway  run  just  north  of 
the  plateau  escarpment  from  Albany  to  Rochester.  (Fig. 
299.) 

From  a  good  map  make  a  list  of  the  streams  that  drain 
(a)  into  the  Atlantic  from  this  plateau,  (b)  into  the  Ohio, 
(c)  into  the  Great  Lakes. 

(e)  The  Adirondack  Mountains  lie  north  of  the  Alle- 
ghany plateau  and  are  separated  from  it  by  the  Mohawk 


Fig.   300.     View    of   the   Adirondack    Mountains   near   Keeseville,    N.   Y.      The 
bordering  plain  in  the  foreground. 

Valley.  The  rocks  of  the  Adirondacks  ally  them  closely  to 
the  New  England  plateau  area  from  which  they  are  sepa- 
rated by  the  Champlain  Valley.  They  are  older  than  the 
Alleghany,  the  Catskill,  or  the  Green  Mountains.  (Fig. 
300.) 


458  PHYSICAL    GEOGRAPHY 

356.  II.  The  Lake  Plains.— Lying  west  of  the  Adi- 
rondacks  and  extending  west  along  the  border  of  the  Great 
Lakes  is  a  strip  of  variable  width  forming  the  lake  plains. 
The  Lake  Ontario  plain  is  sharply  divided  on  the  west 
from  the  Erie  plain  by  the  escarpment  at  the  mouth  of  the 
Niagara  river  gorge,  but  further  east  in  New  York  State 
the  two  plains  merge  into  one.  In  Michigan,  Wisconsin, 
and  Minnesota  in  places  the  plains  are  rugged  and  diversi- 
fied by  many  hills,  the  area  in  places  resembling  the  Pied- 
mont plateau  of  the  Atlantic  region.  The  Lake  Plains  are 
sometimes  divided  for  more  detailed  study  into  (1)  the 
Superior  lowland,  (2)  the  St.  Paul-Madison  upland,  (3) 
the  Green  Bay  lowland,  (4)  the  Michigan-Huron-Erie  low- 
land, (5)  the  Lansing  upland,  (6)  the  Niagara  upland, 
(7)  the  Ontario  lowland. 

357.  III.  The  Mississippi  Valley  Region.— If  one  in- 
cludes in  the  Mississippi  Valley  all  the  area  between  the 
Alleghany  plateau  and  the  Rocky  Mountains  it  forms  one 
of  the  largest  and  most  important  of  the  physiographic 
regions  of  the  United  States  and  one  so  diversified  that  it 
can  be  readily  subdivided  into  a  number  of  minor  areas. 
As  already  stated  there  is  no  sharp  line  of  separation  be- 
tween the  eastern  side  of  the  valley  and  the  Alleghany 
plateau. 

(a)  The  northern  part  of  Indiana,  nearly  all  of  Illi- 
nois, portions  of  Missouri,  Iowa,  Kansas,  Nebraska,  Min- 
nesota, and  the  Dakotas  are  covered  with  great  stretches 
of  treeless  plains  known  as  prairies,  in  some  places  rolling 
in  others  remarkably  level  for  long  distances.  In  the  wild 
state  these  prairies  were  covered  with  tall  grass  and  al- 
most devoid  of  trees.  Since  it  has  been  brought  under 
cultivation  many  orchards  and  groves  have  been  planted. 

(b)  The  western  part  of  the  valley,  extending  from  the 
prairies  to  the  Rocky  Mountains  and  covering  portions  of 


THE    PHYSIOGRAPHIC    REGIONS  459 

Colorado,  Kansas,  Nebraska,  Wyoming,  Montana,  and  the 
Dakotas  is  known  as  the  Great  Western  Plains.  A  large 
part  of  the  area  has  a  semi-arid  climate,  too  dry  for  farm- 
ing (other  than  grazing),  without  irrigation.     Some  por- 


FlG.   301.      View    on     the    (xreat     Western    Plains.       Notice    tlie    clweumg 
house  on  the  horizon.      (U.   S.  Geol.   Survey.) 

tions  of  the  area  are  watered  from  artesian  wells  (sec.  58).' 
Other  portions  are  irrigated  from  water  ponded  in  the 
deep  canyons  of  the  Rocky  Mountains. 

(c)  There  are  several  mountain  areas  over  the  western 
part  of  the  Mississippi  Valley.  The  Washita  Mountains 
south  of  the  Arkansas  river  in  the  states  of  Arkansas  and 
Oklahoma  are  ridge  mountains  similar  in  structure  and 
age  to  the  Alleghany  ridges  of  the  Atlantic  area. 

(d)  The  Boston  Mountains  lie  north  of  the  Arkansas 
river  in  Arkansas  and  Oklahoma.  They  consist  in  part  of 
dissected  plateaus  but  there  has  been  some  folding  and 
faulting  in  places.  They  belong  in  the  same  class  with  the 
Catskill  Mountains,  but  are  somewhat  more  complex. 


460 


PHYSICAL    GEOGRAPHY 


(e)  Covering  a  considerable  area  north  of  the  Boston 
Mountains  is  an  upland  area  known  as  the  Ozark  Plateau. 
The  Boston  Mountains  stand  on  and  form  a  part  of  the 
plateau.     (See  fig.  295.) 

(f)  The  Black  Hills  in  western  Dakota  and  Eastern 
Wyoming  are  dome  mountains  in  the  same  class  with  the 
Adirondacks  but  more  recent  in  age  and  less  complicated 
in  structure.  In  the  area  bordering  the  Black  Hills,  and 
to  a  less  degree  elsewhere  in  the  western  plains,  there  is  a 


Fig.  302.  "Granite  Needles"  near  Harney  Peak  in  the  Black  Hills.  (U.  S. 
Geol.  Survey.)  Part  of  the  area  is  very  rugged.  There  are  other  forms 
of  hills  and  ridges  in  the  Black  Hills  area. 

type  of  topography  kno^\^l  as  the  Bad  Lands.  It  consists 
of  a  very  irregular  surface  with  a  maximum  number  of 
deep  gullies  and  narrow  ridges  with  sometimes  fantastic 
shapes.     (Figs.  303,  39,  40,  41,  and  234.) 

(g)   The  Delta  of  the  Mississippi  is  a  vast  stretch  of 
lowland  covering  a  large  part  of  Louisiana.    It  is  covered 


THE    PHYSIOGRAPHIC    REGIONS 


461 


with  a  network  of  streams,  bayous,  and  lakes.  The  greater 
part  of  the  lowland  area  is  very  fertile  as  it  is  composed  of 
rich  alluvium  carried  down  by  the  Mississippi  river.  The 
water  table  is  so  near  the  surface  that  the  region  never 


Fig.  303.  "Chapoau  do  Ferame"  in  the  Bad  Lands  of  S.  Dakota.  (V.  H. 
Geol.  Survey.)  Columns  40  ft.  high.  Notice  the  effect  of  alternating 
hard  and  soft  layers.  The  erosion  is  partly  by  occasional  heavy  rains, 
partly  by  winds. 

suffers  from  drought  but  sometimes  it  does  from  floods. 
It  is  placed  in  the  Mississippi  Valley  region  but  it  is  just 
as  properly  a  portion  of  the  Gulf  region  as  it  has  all  been 
reclaimed  from  the  Gulf. 

(h)  The  flood  plain  of  the  Mississippi  is  the  lowland 
area  bordering  the  river  that  is  subject  to  periodical  over- 
flow from  floods  in  the  river.  It  varies  in  width  from  a 
few  miles  to  100  miles  or  more. 

358.    IV.     The  Gulf  Region  is  in  part  a  continuation 


THE    PHYSIOGRAPHIC    REGIONB 


463 


of  the  Atlantic  coastal  plain,  consisting  like  it  of  a  sub- 
marine plain,  coastal  marshes,  and  emerged  plain.  The 
latter  extends  north  into  and  partly  forms  the  Alabama- 
Georgia  cuesta*  in  the  east  and  the  Texas  cuesta  in  the  west. 


Fig.   305.     View  on  the  staked  plain  or  cuesta  of  Texas,      (W.  L.  Bray.)      It 
is  an  arid  region  with  little  vegetation  or  surface  water. 

The  Texas  cuesta  is  also  known  as  the  Llano  ^Estacado  or 
staked  plain.  The  staked  plain  is  the  southern  extension 
of  the  Great  Western  Plains  and  like  them  is  too  arid  for 
agriculture  except  when  irrigated.  It  terminates  on  the 
west  at  the  Pecos  Valley.  The  Trans-Pecos  country  be- 
tween the  Rio  Grande  and  the  Pecos  Rivers  contains  the 
San  Francisco  or  Trans-Pecos  Mountains. 


*A  cuesta  is  a  low  ridge  with  a  steep  descent  on  one  side  and  a  gentle 
slope  on  the  other.  On  most  cuestas  the  gentle  slope  is  towards  the  present  or 
former  sea  shore. 


464 


PHYSICAL    GEOGRAPHY 


359.  V.  The  Western  Interior  Area.— (a)  Between 
the  Great  Plains  and  the  Sierra  Nevada  is  a  stretch  of  up- 
land country  composed  of  mountains,  plateaus,  plains,  and 
basins.  The  great  range  of  mountains  bordering  the  plains, 
the  hiofhest  and  most  massive  in  the  United  States  is  com- 


^g^;yt»^.^^_  T^-^ 

Fig.  306.  Outcrop  of  a  great  sandstone  ledge  at  the  end  of  the  Freezeout 
Mountains  in  Central  Wyoming.  One  of  the  subsidiary  mountains  in  the 
Rocky  Mountain  system. 


monly  known  as  the  Rocky  ^fountains.  They  form  the 
backbone  of  the  continent  and  consist  of  great  complexities 
of  mountains  rather  than  a  simple  range.  There  is  a  de- 
pression or  break  in  these  mountains  through  central 
Wyoming  followed  by  the  Union  Pacific  railway,  south  of 
which  the  mountains  have  been  called  the  Park  Mountains 
and  those  north  the  Stony  Mountains.     In  both  areas  there 


THE    PHYSIOGRAPHIC    REGIONS 


465 


are  many  portions  to  which  more  local  names  have  been 
given,  some  of  which  are  the  Gallatin,  Laramie,  Freezeout, 
Elk,  and  San  Juan  Mountains.  Pike's  Peak  at  Manitou 
is  one  of  the  best  known  of  the  many  high  peaks,  and  is 
the  highest  mountain  peak  in  the  world  which  has  a  rail- 
way extending  to  its  summit. 


FiG.  307.  Western  escarpment  in  (he  J-'ict-zcout  Mountains.  ( U.  G.  Cornell). 
A  typical  "Hog  Back"  Ridge,  one  of  the  common  physiographic  features 
of  the  Rocky  Mountain  region. 

The  Hog  Back  ridges  forming  the  foothills  of  the 
Rocky  Mountains  are  characteristic  features  of  this  region. 
They  are  narrow,  sharp-crested  hills  formed  by  the  out- 
cropping edges  of  hard  layers  of  rock  between  softer  layers 
which  were  turned  up  nearly  vertical  in  the  uplift  which 
formed  the  mountains.  They  vary  in  height  from  100  ft. 
to  1,000  ft.  or  more.     (See  figs.  307,  308,  and  35.) 

Numerous  rivers  have  cut  wonderful  canyons  deep  into 
30 


466 


PHYSICAL    GEOGRAPHY 


the  rocky  center   of  these  great  mountains,   exposing  to 
view  many  valuable  veins  of  gold,  silver  and  other  metals. 


i-'ia.  308.  "Cathf'dral  Spires"  in  in.'  ...n.l.n  ui  tin-  (io.ls.  O  •  _^-  ^-'-'l- 
Survey.)  Formed  by  irregular  erosion  on  tlic  outcrop  of  the  vertical  beds 
of  red  sandstone. 


THE    PHYSIOGRAPHIC    REGIONS 


467 


(b)  The  Wahsatch  and  Uintah  Mountains  in  Utah,  the 
Basin  Ranges  in  Nevada,  Idaho,  California  and  Arizona 
are  other  large  and  picturesque  mountains  in  the  interior 
region. 


Fig.  309.  Knight's  Butte,  Central  Wyoming.  Copyright,  1900,  by  U.  G. 
Cornell.  A  view  on  the  dissected  high  plains  in  the  midst  of  the 
Stony  Mountains. 


(c)  The  great  Colorado  Plateau  west  of  the  Park  Moun- 
tains and  the  Columbian  Plateau  west  of  the  Stony  Moun- 
tains are  two  of  the  largest  and  highest  plateaus  in  Amer- 
ica. The  first  is  deeply  trenched  by  the  great  Colorado 
River  and  its  tributaries  and  the  second  by  the  Columbia 
and  Snake  rivers.  In  both  of  these  are  some  of  the  deep- 
est and  most  picturesque  canyons  in  the  world. 

(d)  The  Interior  Bami  lying  between  the  two  plateaus 
and  the  Pacific  mountains  really  consists  of  a  great  number 
of  basins  containing  numerous  lakes,  fresh,  salt,  and  alka- 


468 


PHYSICAL    GEOGRAPHY 


Fig.  310.     Photograph  of  relief  map  of  portion  of  the  Colorado  Plateau  and 
the  Grand  Canyon  of  the  Colorado.      (E.  E.  Howell.) 


line.  The  entire  basin  region  has  an  arid  climate  but  ir- 
rigation has  made  some  portions  of  it  quite  fertile  and 
prosperous.  Much  of  the  area  is  covered  with  plains  of 
sand,  alkali,  or  salt.  (Figs.  312,  274,  275,  79,  80,  and  84.) 
Some  portions  of  this  area  are  the  lowest  on  our  con- 
tinent. The  Salton  basin  in  southern  California  is  287 
feet  below  sea  level  and  Death  Valley  in  eastern  California 


THE    PHYSIOGRAPHIC    REGIONS 


469 


is  276  feet  below.  Both  of  these  areas  belong  by  position 
to  the  Pacific  region  but  the  fact  that  they  are  interior 
basins,  places  them  physiographically  in  the  Western  In- 
terior region.  They  contain  large  deposits  of  salt,  borax, 
soda,  and  other  salts.  A  great  many  mountains  of  the 
class  known  as  block  mountains  occur  in  the  Interior  Basin. 
(See  fig.  312  and  sec.  266.) 


i'lG.  bll.  View  in  the  Grand  Canyon  of  the  Colorado,  the  deepest 
and  one  of  the  most  picturesque  canyons  in  the  world.  (A.  R. 
Crook.) 

360.  VI.  The  Pacific  Area.— In  its  broader  features 
the  Pacific  region  consists  of  two  mountain  ranges  with 
the  Great  Valley  of  the  Pacific  separating  them.  The 
eastern  of  the  two  ranges  is  known  as  the  Sierra  Nevada 
at  the  south  and  the  Cascade  Mountains  farther  north. 
The  western  range  is  called  the  Coast  Mountains.  The 
central  portion  of  the  Great  Valley  is  the  Valley  of  Cali- 
fornia, occupied  in  part  by  the  Sacramento  and  San  Joaquin 
rivers,  the  southern  extension  is  through  the  Salton  Basin 


470 


PHYSICAL    GEOGRAPHY 


into  the  Gulf  of  California.  Northward  it  is  continued 
as  the  Sound  Valley  through  Oregon  and  Washington. 
This  valley  on  the  west  side  of  the  continent  corresponds 
to  the  Great  Valley  of  the  Appalachians  on  the  east  and 
adds  to  the  symmetry  of  the  national  area. 


Fig.  312.     One   of  the  block   mountain  xidges  in  the   Interior  Basin   region. 
(D.  T.  McDougal.) 

This  great  western  valley  is  more  distinctly  divided 
into  sections  than  is  its  eastern  prototype.  The  Klamath 
Mountains  in  Oregon  form  a  mountain  mass  connecting 
the  Sierra  Nevada,  Cascade,  and  Coast  Mountains  across 
the  valley.  In  southern  California  also  the  Sierra  Nevada 
and  Coast  Mountains  join  and  cut  off  the  Salton  Valley 
from  the  Valley  of  California. 


THE    PHYSIOGRAPHIC    REGIONS  47V 

The  student  should  make  a  classified  list  of  the  physiographic 
features  of  the  United  States,  locating  each  by  states  as  follows: 

I.  Plains.      • 

1.  The  coastal  plains. 

2.  The  interior  plains  that  were  at  one  time  coastal 
plains. 

3.  The  cuestas. 

4.  The   prairies. 

5.  Lake  plains. 

6.  Alluvial  plains — deltas. 

II.  Plateaus. 

1.  Arid  plateaus. 

2.  Forested   and  cultivated   plateaus. 

III.  Mountains. 

1.  Folded  ridge  mountains. 

2.  Domed  mountains. 

3.  Mountains   of  erosion. 

4.  Block   mountains. 

5.  Volcanic  mountains. 

IV.  Rivers. 

1.  Which  drain  into  the  Atlantic?  the  Pacific?  Gulf  of 
Mexico? 

2.  Which  fiow  through  deep  canyons? 

3.  Which  have  high  falls? 

4.  Which  are  valuable  aids  in  navigation? 

5.  Which   drain  fertile  farm  land? 

6.  Which  drain  coal  basins? 

7.  Which  have  large  deltas? 

8.  Which  have  none? 

9.  Which  have  no  flood  plains  of  any  size? 

V.  Name  and  locate  the  salt  lakes. 

On  a  blank  map  of  the  United  States  sketch  in  the  boundaries 
of  Lakes  Iroquois,  Passaic,  Agassiz,  Bonneville,  Lahontan  and 
any  other  fossil  lakes  known.    See  references  in  chapter  on  lakes. 


BEFEBENCES 

1.  ;powell,  Physiographic  Regions  of  the  United  States^ 

2.  Davis,  New  England  Plateau. 

3.  Willis   and   Hayes,   Appalachian   Mountains. 


472  PHYSICAL    GEOGRAPHY 

All  of  the  above  published  by  the  American  Book  Co. 

4.  Russell,  Rivers  of  North  America,  G.  P.  Putnam's  Sons. 

5.  Russell,  Volcanoes  of  North  America,  Ginn  &  Co. 

6.  Russell,  North  America,  D.  Appleton  &  Co. 

7.  Hall,  Geography  of  Minnesota,  The  H.  W.  Wilson  Co. 

8.  The   American   Deserts.     National   Geographic    Magazine, 

April,  1904. 

9.  Mill,  The  International   Geography,  D.  Appleton   &   Co. 
10.     Simonds,  Geography  of  Texas,  Ginn  &  Co. 


APPENDIX  I 


473 


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APPENDIX  II 

SOME   OF  THE   COMMON  METHODS  OF  MAP 
PROJECTION 


The  cylindrical  projection  supposes  a  cylinder  of  paper 
around  the  globe  touching  the  equator  and  hence  parallel 
to  the  axis  of  the  globe.  On  this  the  meridians  and  paral- 
lels are  projected  at  right  angles  to  each  other,  the  mer- 
idians forming  vertical  lines  and  the  parallels  horizontal 
ones.  The  meridians  are  equally  spaced  and  all  points  on 
the  equator  are  in  their  true  proportion,  but  toward  the 
poles  the  areas  are  much  out  of  proportion.     If  the  prd- 


FiG.  313.     Cylindrical    projection. 

jection  of  the  parallels  is  from  the  center  of  the  globe,  the 
pole  of  the  earth  is  at  an  infinite  distance  and  cannot  be 
represented ;  also  the  polar  regions  are  greatly  exaggerated 
and  not  represented  beyond  70  or  75  degrees.  If  the  pro- 
jection of  the  parallels  is  at  right  angles  to  the  axis,  the 
polar  regions  are  out  of  proportion  in  the  opposite  direc- 
tion.    (See  fig.  313.) 

Mercator^s  projection  is  a  modified  form  of  the  cylin- 

474 


METHODS    OF    MAP    PROJECTION 


475 


drical,  in  which  the  parallels  are  so  spaced  that  the  degrees 
of  latitude  and  longitude  are  in  their  proper  proportions. 
It  is  much  used  by  navigators  in  plotting  the  course  at 
sea,  because  the  directions  are  all  true  and  the  course  can 
be  plotted  in  a  straight  line.  (Fig.  18,  p.  34,  is  on  Mer- 
cator's  projection.) 

In  the  stereographic  projection,  commonly  used  in  map- 
ping the  hemispheres,  a  sheet  of  paper  is  placed  without 
curving  parallel  to  the  axis  of  the  globe,  touching  the 
equator  at  one  point  in  the  middle  of  the  hemisphere  to  be 
mapped.  The  lines  are  then  projected  on  the  paper  from 
the  point  at  the  opposite  end  of  the  diameter  touched  by 
the  paper. 

The  globular  projection  differs  from  the  stereographic 
in  being  projected  from  a  point  1.707  times  the  radius  of 
the  globe. 

The  orthographic  projection  differs  from  the  preceding 
in  being  projected  from  a  point  at  infinity;  that  is,  the 
lines  of  projection  pass  through  the  globe  parallel  to  each 
other  and  normal  to  the  paper. 


3 

k 

^ 

%K 

>^ 

\^^ 

/ 

/^ 

^  Globe. 

\____3^ 

Fig.   314.      Conical  projection. 


The  conical  projection  assumes  a  cone  touching  the 
earth  on  the  parallel  passing  through  the  middle  of  the 
area  to  be  mapped  and  the  lines  projected  on  the  cone  from 


476 


PHYSICAL    GEOGRAPHY 


the  center  of  the  globe.  The  cone  is  then  split  open  on  a 
meridian  line  and  spread  out  flat.  This  is  more  accurate 
than  any  of  the  preceding  for  small  areas  away  from  the 
equator.  In  large  areas  the  distortion  becomes  pronounced 
away  from  the  center  of  the  map ;  but  where  greater  accu- 
racy is  required  this  defect  is  sometimes  remedied  in  part 
by  using  a  polyconic  projection. 


Fia.   315.      Polar  projection. 


In  the  polar  projection,  as  shown  in  fig.  315,  the  paper 
is  placed  tangent  to  the  pole  and  from  the  center  of  the 
globe,  one  point  on  each  parallel  is  projected  to  the  paper 
as  at  P,  Q,  R.     With  N  (the  pole)  as  a  center,  circles  are 


METHODS    OF   MAP    PROJECTION  477 

drawn  through  these  points  for  the  parallels.     Radial  lines 
from  the  center  (N)  form  the  meridians. 

Polar 


Fig.   316.      Projections  illustrated  with 
wire   screen. 


In  none  of  the  above  projections  is  a  globe  used  in 
actual  construction,  but  the  lines  are  located  by  com- 
putation. 


GENERAL  REFERENCES 

Special  references  are  given  at  the  end  of  each  chapter. 
The  following  general  reference  books  contain  valuable 
data  on  different  phases  of  the  subject  and  should  be  con- 
sulted as  far  as  possible  by  both  teacher  and  students: 

1.  Physiography  by  R.  D.  Salisbury,  Henry  Holt  &  Co. 

2.  Text  Book  on  Geology,  3  Vols.,  by  Chamberlin  and  Salis- 
bury, Henry  Holt  &  Co. 

3.  Other  text  books  on  Physical  Geography  and  Geology. 

4.  The  International  Geography  by  70  authors,  D.  Appleton 
&  Co. 

5.  Proceedings  of  the  8th  International  Geographical  Con- 
gress, Washington,  1904. 

6'.  Publications  of  the  U.  S.  Geological  Survey  consisting  of 
Geologic  and  Topographic  Atlas,  Monographs,  Bulletins,  Profes- 
sional Papers,  Annual  Reports,  and  Water  Supply  and  Irrigation 
Papers. 

The  contour  maps  of  the  Topographic  Atlas  are  of  special 
importance.  They  can  be  obtained  from  the  Director  of  the  U.  S, 
Geological  Survey,  Washington,  D.  C,  at  five  cents  each  or  three 
dollars  per  hundred. 

PERIODICALS 

The  National  Geographic  Magazine,  Washington,  D.  C. 

The  Journal  of  Geography,  N.  Y. 

The  Bulletin  of  the  American  Geographical  Society,  N.  Y. 

School  Science  and  Mathematics,  Chicago,  111. 

The  Journal  of  Geology,  Chicago,  111. 


478 


INDEX 


Abyssal  life,  437 

Adirondack    Mountains,457 

Aggrading,   71 

Aletsch  Glacier,   139 

Alkali  plains,   317 

Alkaline  lakes,   113 

Alleghany  plateau,   93,   327,  454 

Alluvial  cone,  87 

fan,   86 

plain,    313 

soil,    263,    269 
Alpine  glaciers,    141 
Aluminium  ores,    250 
American  Museum  of  Natural  History, 

11,    282,    283,    284 
Andromeda    nebula,    14 
Anemometer,    379 
Aneroid  barometer,  353 
Antecedent  river,   94 
Anticline,    337 
Aphelion,   22 
Aquifer,    42,    55,    56 
Arid  climate,   97,   98 
Arroyo,   98,   330,   331 
Artesian  well,   55,   311 
Atmosphere,    348 
Atoll,    222 
Augite,   244 
Aurora    borealis,    396 
Ausable  Chasm,    73,    322 

Bad  Lands,    65,   330,    461 

Barnett  Falls,    131 

Barogram,    354 

Barograph,    355 

Barometer,    352 

Barriers,    212,    344,    417,    431 

Bars,  209 

Base  level,   71 

Bates'  Hole,    65 

Beach,    208 

Beaver    lakes,    105 

Biela's  comet,    10 

Big  trees,    412,    429 


Black  Hills,   460 
Blind  fish,  43 

Block  mountains,  341,   470 
Bogs,    124 
Bore,  tidal,    183 
Boston  Mountains,  459 
Boulder  clay,    148,    149 
Boulders,    158,    160,    161 
Breakers,    177 
Breakwater,   233 
Breccia,    257 
Building  stone,   346 
Buttes,    323,    324,    462 

Calcite,  244    . 

Calderas,  295 

Calendar,  29 

Calories,    357 

Canoe  mountains,    339,    340,   454 

Canyons,    322 

Caroline   Bridge,    48 

Catskill    Mountains,    455 

Cave  deposits,    50 

Caves,    42 

Chamberlin,    T,  C,   13 

Charleston  earthquake,   300 

Chimney  rocks,    204 

Chinook,  377 

Cirques,    140 

Clay,   257 

Cliff   glaciers,    141 

Climate,    288,    394,    400,    441 

Cloudburst,   387 

Clouds,    363,    382-385 

Coal,    258 

Coastal   plain,    310,    312,    313,    355 

Cold  wave,    377 

Colorado  plateau,  467 

Composition  of  earth's  crust,  238 

of    atmosphere,     349 
Continental  glaciers,    141 

shelf,  169 
Continents,  236 
Contour  maps,  36 


479 


480 


INDEX 


Copper  ores,    249 
Coral,    218 
Coal  Creek,  80 
Coral  reefs,    221 

harbors,    232 
Corrasion,   73,    152 
Crater    Lake,    103 
Creep,    263,    264 
Crevasse,  river,  82 

glacial,    145 
Crouse  Boulder,   158 
Crystals,   241 
Cuesta,    463 
Cycle  of  erosion,  87 
Cyclones,    370,    371,    386,   889 
Cypress,   402 

Daniel's  comet,    10 

Day,    30,    31 

Deeps,    174 

Deep  sea  deposits,    189,   190 

life,    194,    436,    437 

oozes,    190 
Degrading  by    streams,    70 
Deltas,  84,   101,   114,   460 
Delta  harbors,    230^^ 
Desert  plants,  407 
Deserts,    328-335,    419 
Dew,    381 
Dew  polnr    ^.SO 
Diastrophism,    274 
Diatoms,    117,    118,    119,    190 
Dikes,    205,    297 
Diorite,    260 
Directions,   20 
Disintegration  of  rock,    262 
Distributaries,    84,    314 
Divides,  migration  of,   95 
Dolomite,    245 
Domed    mountains,    340 
Dredges,    172,   173 
Drift,    glacial,    149 

oceanic,    187 
Drumlin,    150 
Dust,    351 

Earth,  a  magnet,  33 
motions  of,  19 
origin  of,    11  , 

part  of  solar  system,   2 
revolution  of,  21 


rota,tion   of,    20  , 

shape  of,    16,   17 

size   of,    17 

structure  of,  19 

shine,  5 
Earthquake   waves,    178 
Earthquakes,    104,    299-307 
Eel  grass,  218 
Eclipses,   7 

Economic    features    of    coastal    plains 
313 

of   glaciers,    167 

of  harbors,  232 

of  mountains,   345 

of  the  ocean,   195 

of  plateaus,    325 

of  swamps  and  marshes,   125 
Ellipse,    9 

Engrafted   rivers,    94 
Epeirogenic  movement,   275 
Epiphytes,   403 
Eratosthenes,    18 
Esker,   150 
Eskimo,   443 
iiiureka  Springs,  59 

Fall  line,   135 

Falls,    125 

Faults,    322,    325 

Feldspar,    242 

Ferrel's   law,    187 

Fiord  harbors,    231 

Fissures,    296 

Flood   plains,    78,    123,   315,    461 

Fluorite,    254 

Forests,    423-430 

Fossil,   shore  lines,    224 

lakes,    116 

reefs,    222 
Foucault's  pendulum,  20 
Frost,  381 

Gabbro,   260 

Garden  of  the  Gods,    160,   466 

Gegenshein,    398 

Geographic    cycle,   270 

Geysers,    61 

Glacial   channels,    162,    163 

plains,    317 

soils,    269 
Glaciers,   138-167 


INDEX 


481 


Glaciers,   economic   effects   of,    167 
movements    of,    143-164 
North  American,    165 

Glauconite,   191 

Graded   streams,    71 

Grand  Canyon,   323,  468,   469 

Granite,   259,  260 

Graphite,    253 

Gravitation,   17 

Great   Interior   Basin,    112 

Great   Lakes,    111 

Great  Salt  Lake,  108,  112,   214,  225 

Great  Valley,    248,  444,  454 

Groundwater,   41,    53 

Gulf   Stream,    185 

Gypsum,  252 

Halite,   250 
Halley's    comet,    10 
Hanging  valleys,    156,    157 
Harbors,   228-234 
Hardness,  scale  of,  240 
Hematite,    246 
Hog  Back   Mountains,    465 
Hook,    209    . 
Hornblende,    244 
Horse   latitude,    370 
Hot  springs,    61 
Humidity,   379 
Hurricane,    297 
Hygrodeik,   381 
Hygrometer,    380-381 
Hyperbola,    9 

Icebergs,    160 

Ice  tables  and  pinnacles,   146 

Indus  River,    85 

Insolation,    6 

International  date  line,  31 

Iron  ores,    246 

Iroquois   Lake,    116 

Islands,    236 

Isobars,    356 

Isoclinal   lines,    33 

Isogonic  lines,  33 

Isostacy,    277 

Isotherms,   364 

Kame,  149 
Kaolin,  245 
Karsten,  46 


Kettle  holes,    149 
Kingston    earthquake,    305 

Laccolites,   297,   341 
Lacustrine  plains,    316 
Lakes,    100-137,   224 

disappearance  of,    114 
function  of,    119 
in  arid  regions,   121 
levels,   113 
origin  of,    100 
shores,   224 
Land,    235 
Latitude,    24,   25 
Lava,    289 
Lead  ores,   249 
Levee,   82 
Levee  lakes,  84 
Life  history  of  lakes,   120 
of  a  land  area,   270 
of    mountains,    342 
of  a  river,   87 

of    sedimentary    rocks,    269 
of    a   volcano,    296 
Life  in  caves,    43 

in  lakes  and  rivers,   117 
Life  zones,   414 
Lighter,    233 
Lightning,    395 
Limestone,    223,   258 
Limonite,    246 
Lisbon  earthquake,   306 
Longitude,    24,    27,   28 
Lost   River,    45 

Magnesite,    254 

Magnetism,    32 

Magnetite,    248 

Mammoth  Cave,  44 

Man,    distribution  of,    439 

Mangrove,    217 

Mantle  rock,    264 

Maps,   35,   36,  474 

Marble,   258,  261 

Marengo  Cave,  51 

Marl,    114 

Marshes,    123 

Maturity  of  topography,  271 

Meanders,    79,   80 

Mediterranean  seas,   170 

Mercator's  projection,    474 


482 


INDEX 


Mesa,    323 

Metamorphic  rocks,  260 

Meteorites,    10 

Meteors,    10 

Mica,   243 

Minerals,  239 

Mineral    springs,    60 

Mirage,   397 

Mississippi   River,    458,     81,     83,     84, 

315 
Mississippi  valley  earthquake,    299 
Missouri  River,    77 
Monadnocks,  271,   272,  318 
Monsoon,    377 
Moraines,   147 
Mount  Pelee,    281 
Mount  Potosi,  67 
Mount  Vesuvius,  278,  279 
Mountains,    262,    335,    346,    419,    442 
Muck,   268 
Muirs  Butte,  292 

Nadir,  21 

Natural  Bridge,   46,  206,  207 

Nebula,    12,    13 

Nebular  hypothesis,    12 

Neve,    140 

Niagara   Falls,    126,    127 

Northeaster,   377 

North  Platte  River,  88 

Obsidian,   260 

Ocean,    169 

Ocean   life   in,    192,    194 

currents,    185 
Onyx  marble,   51 
Oozes,   190 
Ores,    246 

Orogenic  movement,    275,    336 
Ouray,  Colo.,   86,    142,   153,   154,   155 
Overloaded  stream,  76,  77 
Ox-bow  lakes,    81 
Oxygen,  349 

Parabola,  9 

Peat,    115 

Pelagic  life,    194,    438 

Peneplain,  271,  318 

Perched  boulders,    158 

Perihelion,    22 

Phases   of   moon,    5 


Physical  Geography,  1 
Physiographic   agencies,    274 

features,    309 

regions,  449 
Piedmont   glaciers,    141 

plateau,    452 
Pigmies,    443 
Pilaster,     51 
Pittsburg,  444,  445 
Plains,   309,    419 
Planetoids,   2,    3 
Planets,    2 

Planetesimal  hypothesis,   13 
Plant   Geography,    402 
Plateaus,  321 
Playas,    98 

Polar  projection,   476 
Pot-hole,    73,    155 
Potomac   River,    70 
Prairies,    320,    411 
Precipitation,    385 
Projections,    35,   474 
Pumice,    260 
Pyrite,    248 

Quaking  bogs,    123 
Quartz,  240 
Quartzite,   261 

Rainbow,   397 
Rainfall,    40,    386 
Rain   gauge,    385 
Rapids,    69 
Reaches,    69,   134 
Reelsfoot  Lake,   104 
References,    39,    99,    137,     167,    196, 
234,  273,  307,   346,  398,  447,  471, 
478 
Residual  soil,  267,   268 
Reversed  drainage,    93 

rivers,   92 
Revolution  of  the  earth,  21 
River  deposits,   77 

profile,    68,    69 

piracy,    95 

swamp,   83 

valley,    64 
Rivers,  40,  63,  67,  72 
Rocking    stones,    158 
Rocks,    254 
Rocky   Mountains,    464 


INDEX 


483 


Saint   Elmo's  fire,    396 

Salinas,    112 

Salt,    250 

Salt  lakes.    111,   214 

marshes,   124 

plains,    317 
Salts  of  the  ocean,    171 
Salton   sink,    107,    108 
Sand  dunes,  226 
Sandstone,    256 

San   Francisco  earthquake,    302-305 
Sargasso    seas,    186 
Sargassum,   407 
Satellite,  4 
Sea  caves,   206 
Sea  water,  composition  of,  170 

density  of,    172 

depth  of,    174 

temperature  of,    175 
Seasons,    22 

Sedimentary  rocks,   255 
Seepage,  60 
Seiche,    114 
Shale,   257 
Shooting   stars,    10 
Shore  cliff,    203 

lines,    197-234,    277 

terraces,   213 
Sink  holes,    45,    106 
Snow  fields,    138 
Soil,    262,    402 
Solar  day,   30 

system,    2,    3,    473 

time,    30 
Sounding  and   dredging,   172 
Spit,   209 

Spouting  caves,  208 
Springs,    51,    58 
Stalactite,    50 
Stalagmite,    50 
Standard   time,    31 
Subsequent  streams,   94 
Sulphur,    252 
Superimposed  rivers,    92 
Swamps,   123 
Syenite,    260 
Syncline,    337 

Talc,    253 
Talus  cone,   87 
Taughannock  Creek,    78 


Temperature,    357,    360,    413 

Tent  meteorite,   11 

Terminator,    5 

Terraced  mountains,  338 

Terraces,   92,   93,    123 

Thermogram,    358 

Thermograph,    359 

Thermometer,  357 

Thunderstorm,   387 

Tides,    181,   184 

Till,    149 

Timber  line,  4 Iff 

Time,   29 

Tinker's  Falls,    134 

Toad  Stool  Park,    66 

Topogi;^phic  atlas,   38,   478 

Topography  of  ocean  bottom,  188,  238 

of  shore  lines,    197,   203 
Tornado,    375,    376 
Transportation  by  rivers,  74,  75 
Trade  wind,    368 
Travertine,    51 
Tripoli,    118 
Tufa,   51,  290 
Ttindras,    321 

Underloaded  stream,  76 
Undertow,    178 
Uncompaghre    Creek,    76 
Universe,    3 

Vegetation,    402 

Vegetation  on  the  shore  line,  217 
Veins,   48,    49 
Volcanic   harbor,    231 
mountains,     341 
Volcano,  279,   287,  295 

Wadies,    97 

Water,  •  effect  on  life,    403 

Water  gaps,    94 

plants,    404 

table,   41,    54 

zone,    41 
Waterspout,   376 
Watkins   Glen,    73 
Waves,  176,   179,   181,  199,  200 
Weather,  388 
Weather  forecast,  393 

map,    390 


484 


INDEX 


Wells,   artesian,   55 

common,    54 
Whistling  caves,    208 
Willamette  meteorite,  11 
Wind,    366,  368-379 
Wind  gaps,   94 
Work  of  rivers,  72 


Wyandotte  Cave,    52 

Youth  of  a  river,   87 

Zinc  ores,   259 

Zodiacal   light,    398 

Zones,   366,   367,    393,  413 

Zoological   provinces,    430,    431,    432 


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DEC  26  1924 

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JAN  : 


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